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Improving Email Deliverability

Learn how to enhance email deliverability by following essential authentication and content best practices.

Ruba

10 Feb, 2026 - Updated 20 days ago

Click here to watch the video on email deliverability

Introduction

Email is a powerful tool for engaging customers and building relationships. However, spam filters play a crucial role in determining whether your emails land in the inbox or get flagged as spam. Emails often end up in spam due to poor authentication or spammy content.

If your emails are going to spam, it could be a deliverability issue. Let’s look at what that means.

Email Deliverability

Email deliverability refers to the ability of an email to reach the recipient’s inbox successfully. Factors such as sender reputation, email content, and authentication protocols affect deliverability. If these factors are not met, the email may end up in the spam folder instead of the inbox.

For instance, a poorly formatted email with broken HTML and excessive trigger words like free or urgent is more likely to land in the spam folder than a well-structured, personalized message.

Let’s take a closer look at what spam filters and spam folders are, and how they impact email delivery.

Spam Folder

A spam folder is a part of your email where unwanted or suspicious emails are sent automatically by spam filters. Emails identified as potential spam or failing to meet certain security or authenticity standards end up in this folder instead of the inbox.

However, even valid emails can mistakenly land in the spam folder if they trigger spam filters due to poor sender reputation, improper authentication, or spammy content.

Spam Filter

A spam filter is an automated algorithm used by email providers like Gmail, Outlook, and Yahoo to protect users from unwanted or harmful emails. These filters analyze incoming messages for specific patterns, such as suspicious links, excessive use of spam trigger words, or improper authentication.

Spam filters play a crucial role in controlling inbox placement and protecting users, but they can also block genuine emails if best practices are not followed.

Why Emails Land in Spam?

Emails rarely end up in spam for a single reason. Instead, Internet Service Providers (ISPs) evaluate a combination of technical and behavioral signals, such as:

Missing or misconfigured authentication (SPF, DKIM, DMARC)
Poor sender or domain reputation
High bounce or spam complaint rates
Low recipient engagement
Spam‑like content patterns
Sudden or irregular spikes in sending volume
Sending to inactive, invalid, or unconsented contacts

Even legitimate emails can be flagged if these signals indicate risk or low trust.

Benefits of ensuring your emails are delivered without triggering spam filters

The following are the benefits of ensuring your emails are delivered without triggering spam filters:

Improved Inbox Placement

Emails reach your intended audience without being blocked or flagged as spam.
Avoiding spam folders ensures better communication with your recipients.

Higher Engagement Rates

When emails are delivered to the inbox, recipients are more likely to open, read, and engage with your content.
This can increase clicks, responses, and overall campaign effectiveness.

Better Sender Reputation

Consistently landing in the inbox boosts your sender reputation, which helps your future emails avoid spam filters.
A strong sender reputation ensures long-term email marketing success and positively affects your server's reputation with email providers.

Increased Brand Reputation

Implementing email authentication records like SPF, DKIM, and DMARC assures recipients that your emails are genuine and secure.
A professional and trustworthy email system reflects positively on your brand.

In this article, you will learn about:

Best practices to improve email deliverability
Email sending guidelines for Google, Microsoft, Yahoo, and Apple
Analyzing Email Performance
Troubleshooting common email issues

Key Terminology

Key Term	Description
Sender Policy Framework(SPF)	A security method that verifies which mail servers are allowed to send emails on behalf of a domain.
Domain-based Message Authentication, Reporting, and Conformance(DMARC)	An email authentication policy that helps protect domains from spoofing and phishing by using SPF and DKIM results.
Blacklisting	A process where email servers or IP addresses are blocked from sending emails due to spam or suspicious activity.
Whitelisting	A process of marking trusted senders so their emails are always allowed to reach the inbox.

Best Practices to Improve Email Deliverability

Vtiger ensures high email deliverability by supporting essential email authentication standards such as SPF, DKIM, CNAME, DMARC, and SenderID, along with built-in tools to monitor your email performance and sender reputation.
To ensure your emails reach the inbox and not the spam folder, follow these best practices:

Infrastructure and Authentication

A strong technical setup helps mailbox providers verify your identity and protects recipients from spoofed or fraudulent emails.

Authenticate your emails (SPF, DKIM, DMARC)
Email authentication protocols such as Sender Policy Framework (SPF) and DomainKeys Identified Mail (DKIM) help Internet Service Providers ensure that the email is from the company claiming to have sent it.

Authentication helps address the problems of email spoofing and phishing, ultimately leading to a reduction in the percentage of legitimate emails mistaken for spam.

Vtiger CRM supports SPF and DKIM authentication. To learn more about email authentication, click here.

Use a custom sending domain
Avoid sending emails from free email addresses such as Gmail or Yahoo. Using a branded domain improves credibility and reduces the likelihood of spam filtering.

Use app passwords for SMTP authentication
When connecting third-party applications to Gmail or Yahoo Mail as your outgoing (SMTP) email server, always use App Passwords instead of your regular account password for better security and compliance with modern authentication protocols.

Gmail and Yahoo have enhanced security standards that block less secure apps from accessing your account with just a regular password.
App Passwords are unique, system-generated 16-digit codes that allow secure access without exposing your main account credentials.

To learn more about configuring app passwords for the outgoing server, click here.

Avoid using the same “From” and “To” address
Emails sent from and to the same address may appear suspicious and can negatively impact deliverability.

Use Email Validation Tools

Run your mailing list through an email validation service before sending it to reduce the risk of undeliverable addresses.

If you require more details about a specific bounce listed in your report, feel free to reach out to us at [email protected]. Our team will review the bounce logs, check the returned DSN (Delivery Status Notification), and assist you with further investigation.

Reputation Management and Sending Behavior

Mailbox providers closely monitor how you send emails over time to assess sender reliability.

Maintain consistent sending patterns
Too many emails can reduce your subscriber list. Firing fewer emails can kill your revenue. Find the right frequency by testing thoroughly.

A good benchmark is one email per week. You can try scaling up to twice weekly, as long as you have great content.
You can even drop back to once a month. But if you exceed either of these two boundaries, you might lose your audience.
Random and unusual email activity leads to lower sender scores and IP blacklisting.
Avoid sending spikes by maintaining regular email schedules.
Deliver the best for consistent outbound email scheduling.

To learn more about this, click here.

Respect the provider's sending limits
Exceeding daily or hourly limits can result in throttling or temporary blocking. Always warm up new domains or IPs gradually.

Monitor blacklists

Regularly verify if your reply-to email address, sending IPs, or sending domains are listed on email blacklists using tools like MXToolbox.
Send emails through a trusted email provider with a good IP reputation to reduce the chances of blacklisting.

Analyze bounces and delivery errors

Investigate general bounces to identify recurring issues.
If the reason for the bounce isn’t clear, you can try sending the email again after some time, but avoid sending it too many times to the same address.
Regularly monitor email error codes to quickly identify and resolve delivery issues. These error codes provide important clues about why an email failed to reach its recipient—for example, due to authentication issues, full inboxes, invalid addresses, or blacklisting.

To learn more about error codes, click here.

Encourage Recipients to Whitelist Your Domain

Whitelisting ensures future emails bypass spam filtering and reach the inbox.

List Hygiene, Consent, and Compliance

Sending emails only to users who explicitly want them is one of the strongest signals of good deliverability.

Use permission-based and double opt-in lists
The difference between double and single opt-in is that with double opt-in (also called confirmed opt-in), people receive a confirmation email after submitting the form. They are not subscribed until they click the confirmation link they receive in their email.

The following are the advantages of using double-opt-in options:

Deliverability rates for double-opt-in lists are significantly higher than for single-opt-in lists.
Unsubscribe rates are lower than the open and click-through rates.
Double opt-in lists reduce spam complaints.

Note: You cannot use purchased or rented lists in Vtiger CRM to send emails, as this violates the CAN-SPAM Act.

How double opt-in works:

A user submits a signup form
A confirmation email is sent
The subscription is activated only after confirmation

This approach reduces spam complaints, improves engagement, and strengthens sender reputation.

Avoid purchased or rented email lists
These lists often contain invalid or uninterested recipients and can severely damage deliverability.

Remove inactive subscribers
Contacts who haven’t engaged with your emails for several months can negatively impact inbox placement. Periodic cleanup improves overall performance.

Always include an unsubscribe option
Include unsubscribe links in your emails to improve deliverability rates. If the unsubscribe link is unavailable or misplaced, there is a chance that emails will be marked as spam

Remove subscribers who haven’t opened or clicked in six months

If people do not open or click on your emails, removing them from your list might hurt, but it will help improve the deliverability rates for the people you keep.

Include a physical address in your email

Physical address means the mailing or postal address of your business. You can put it at the end of your emails; that is the footer.
Including a physical address in your emails demonstrates that your business is legitimate. Thus, it prevents your emails from landing in the spam folder.

To learn more about including a physical address, click here.

Review Quarantined Emails Regularly

To ensure important emails aren't missed, regularly check the Quarantine folder in Microsoft Outlook or through the Microsoft 365 Security Center.

Review quarantined messages daily or weekly to identify any legitimate emails mistakenly marked as spam or phishing.
Use the Report or Not Junk option to train Outlook’s filters and reduce future false positives.
Release safe emails promptly to your inbox to avoid delays in communication.
Block or report malicious emails directly from the quarantine portal to enhance overall security.

To learn more about managing quarantine emails, click here.

Comply with email regulations (GDPR and CAN-SPAM)

GDPR requires explicit consent, transparency in data usage, and allows recipients to withdraw consent at any time.
CAN-SPAM mandates accurate sender information, honest subject lines, and a working unsubscribe mechanism.

Compliance not only avoids legal issues but also strengthens sender credibility with mailbox providers. To learn more about GDPR, click here.

Content and Engagement Optimization

Mailbox providers analyze both email content and how recipients interact with it.

Avoid spam-triggering language and formatting
Excessive capitalization, misleading phrases, and common spam words can increase filtering risk.

Avoid sending emails with only images or graphics
Emails with a single graphic or image are sure to have delivery issues.

You should balance your emails with images and text content.
It is recommended to write at least two lines of text for every image.
Include plain text along with HTML in emails to avoid spam and support non-HTML viewers.

Avoid Large or Risky Attachments

Avoid executable file types such as .exe, .zip, or .swf. Stick to commonly accepted formats such as .pdf or .jpg.

Avoid copying content from external editors
Copy-pasting from tools like Word or PowerPoint may introduce hidden formatting. Creating templates directly in the email editor ensures cleaner HTML.

Be mindful of links and URLs

Spam filters check the URLs in the email. Linking to a domain that has a poor reputation leads to penalization.
Additionally, you must avoid linking to URLs that contain folders with 1-2 characters (such as domain.com/e/something/ or domain.com/es/) because some spam filters block such URLs.
Avoid including links that utilize link shortening services, as this may cause delivery issues.

Personalize content where possible
Relevant and personalized emails drive higher engagement, which positively influences inbox placement.

Monitor engagement metrics

Track open rates, click-through rates (CTR), bounce rates, and spam complaints.
Remove unengaged recipients after a certain period to keep the list quality high.

Test emails before sending

Use tools like MailTester or AboutMy.Email to check your email for potential spam triggers. These tools analyze your content, provide a score, and offer suggestions for improvement.
Testing ensures your email is optimized for deliverability and avoids landing in spam folders.

To learn more about spam score, click here.

Email Sending Guidelines for Google, Microsoft, Yahoo, and Apple

To improve your email deliverability and avoid being flagged as spam, it’s important to follow the sending guidelines set by major email providers. Below are some general best practices recommended by Google, Microsoft, Yahoo, and Apple:

Compliant P2 (Primary) Sender Addresses: Ensure the From or Reply‐To address is valid, reflects the true sending domain, and can receive replies.
Functional Unsubscribe Links: Provide an easy, clearly visible way for recipients to opt out of further messages, particularly for marketing or bulk mail.
Easy Unsubscription
- One-click unsubscribe for email recipients from large senders.
- Process unsubscription requests within two days.
Prevent Unwanted Emails
- Enforce a spam rate threshold to reduce inbox clutter.
- Maintain a spam-free mailbox for users, as Google has mandated that an email sender’s spam rate should be below 0.3%
List Hygiene & Bounce Management: Remove invalid addresses regularly to reduce spam complaints, bounces, and wasted messages.
Transparent Mailing Practices: Use accurate subject lines, avoid deceptive headers, and ensure your recipients have consented to receive your messages.
Configure Your DMARC Policy: Set up a DMARC policy on your root domain via your DNS provider. Follow these steps to configure your DMARC policy:
- Publish a DMARC record (the policy can be p=none).
- Ensure alignment with their SPF and DKIM domains.
- Implement a DMARC reporting solution (recommended).

To learn more about email sending guidelines, click here.

Monitoring Email Performance Metrics

Regularly analyzing your email performance is essential for maintaining good deliverability and boosting engagement. Key metrics to focus on include:

Open Rate: Shows how many recipients opened your emails. A low rate may point to ineffective subject lines or low sender credibility.
Click-Through Rate (CTR): Tracks the number of people who clicked on links in your emails, indicating how engaging and relevant your content is.
Bounce Rate: Highlights emails that couldn't be delivered. A high bounce rate could mean your contact list needs cleaning.
Spam Reports: Measures how many recipients marked your emails as spam. Submitting too many reports can harm your sender's reputation.
Unsubscribe Rate: Indicates whether your content is relevant. A high rate may suggest you're not meeting your audience's expectations.

Additionally, be sure to check your spam score regularly to prevent your emails from being directed to spam folders. To learn more about spam score, click here.

Troubleshooting Tips

Users often experience the following problems with email. Here’s how you can address them.

Emails sent from the inbox are not delivered to the recipient’s inbox.

Users who haven't added Vtiger as a trusted application in G Suite will experience this problem. To resolve this, you'll need to whitelist Vtiger in G Suite.

SMTP connect() failed error

The system shows an SMTP connect() failed message after entering SMTP details under Other server type and attempting to save. To confirm that your email credentials and server settings are correct, test them in Thunderbird. Follow the linked guide to fix the issue.

Emails are bouncing, and the bounce message indicates the recipient’s server is blocking them due to authentication problems.

The bounce message suggests that the recipient server is blocking the message due to authentication issues. Please verify that your domain’s SPF, DKIM, and DMARC records are properly configured to authorize Vtiger as a sender. This helps maintain deliverability and protects your domain reputation.

Emails sent through Vtiger are being flagged as spam or not reaching recipients using Outlook.

To improve deliverability and avoid being flagged as spam in Outlook, follow these tips:

Authenticate Your Domain: Set up SPF and DKIM records to verify the authenticity of your emails.
Simplify Email Formatting: Avoid complex HTML and excessive images, as Outlook may flag such content.
Avoid Excessive Links: Minimize the number of links in your emails to reduce suspicion.
Minimize Spammy Words: Avoid trigger words like free, urgent, or act now in your subject lines and content.
Request Whitelisting: Encourage recipients to add your email address to their safe sender's list.
Monitor Feedback Loops: Use Outlook’s feedback loop system to track if users are marking your emails as spam.
Use Consistent Sending Patterns: Irregular sending patterns may cause Outlook to flag your emails. Maintain consistent sending volumes.
Leverage Microsoft Smart Network Data Services (SNDS): Use SNDS to monitor sender reputation and identify potential issues.
Test Before Sending: Test your emails to ensure they meet Outlook’s formatting and deliverability standards.

Emails are bouncing with a relay error message.

Relay refers to the transmission of an email from one server to another. Your email has bounced due to a temporary error, which could be on the sending or the receiving side.

This type of bounce usually occurs when the sender’s message is not authenticated or due to a misconfigured server on the recipient side.

To know more about Email Bounces and Error Codes, click here.

Emails from our domain are going to the Junk or Spam folder in Outlook

Some users report that emails sent from our domain are not appearing in their inbox and are instead going to the Junk or Spam folder in Microsoft Outlook.