Being exposed to email security risks is something we all want to avoid; security risks such as man-in-the-middle (MITM) attacks, where attackers intercept and manipulate email traffic, are the bane of all email marketer’s existence.
There are the basics of ensuring a configured DNS and SMTP setting, but some errors go deeper than that.
Some email users receive errors such as:
Authentication failure [SMTP: STARTTLS failed (code: 220, response: TLS go ahead)]
An error that stemmed from when an email client or server fails to establish a secure TLS connection — a security protocol that preserves privacy and data security.
Conducting an email deliverability test is a contributing factor allowing email users to perform proper evaluation and avoid some common and complex errors that are affecting their email deliverability.
However, we at Warmy.io understand that it would take a lot more to avoid such errors, which is why we’ve developed a variety of tools to improve the overall email process.
But first, let us understand what STARTTL is, and what are its repercussions that affect your overall email experience?
What is STARTTLS?
STARTTLS is used to upgrade a plain-text email connection to a secure, encrypted connection using TLS (Transport Layer Security).
It is a protocol common that helps boost email security by preventing unauthorized access to transmitted messages.
STARTTLS works by:
- Connecting an email client to an SMTP server via an unencrypted connection.
- The client issues the STARTTLS command, requesting encryption.
- If the server supports TLS, it upgrades the connection to a secure one.
- Emails are transmitted securely, preventing data interception.
STARTTLS ensures confidentiality and integrity in email communication, making it essential for modern email security practices.
What Causes SMTP STARTTLS Failed Error?
STARTTLS failures occur when the encryption process is disrupted due to misconfigurations or security restrictions. Below are the most common causes:
1. SSL/TLS Certificate Mismatch
- Certificate mismatches leading to authentication failures.
- The email server’s SSL/TLS certificate does not match the domain name.
- Trust issues and connection failures due to expired or self-signed certificates
- Incorrect Common Name (CN) or Subject Alternative Name (SAN) settings in the certificate.
2. SMTP Configuration Errors
- The mail client is configured to use an incorrect encryption type (e.g., forcing SSL instead of STARTTLS).
- The SMTP server is not properly configured to support STARTTLS.
- The mail server does not advertise STARTTLS support, preventing a secure connection.
3. Port Restrictions
- Firewalls or ISPs may block SMTP ports required for STARTTLS.
- Common ports used for secure SMTP:
- Port 25 (Blocked by many ISPs to prevent spam relay)
- Port 465 (Deprecated but still used for SSL/TLS encryption)
- Port 587 (Recommended for secure email sending with STARTTLS)
- If the correct port is blocked, email clients will fail to establish a secure connection.
Fixing SMTP STARTTLS Failed Error
Resolving STARTTLS errors requires addressing configuration issues, updating security certificates, and verifying network accessibility.
1. Correcting SMTP Settings
- Use the correct SMTP port: Ensure that the email client is configured to use Port 587 for STARTTLS.
- Check email authentication settings: Ensure proper login credentials and SMTP authentication (AUTH LOGIN) are enabled.
Verify STARTTLS support: Use command-line tools like openssl to test the server:
openssl s_client -starttls smtp -connect mail.example.com:587
2. Correcting Firewall and Network Restrictions
- Allow SMTP traffic on Ports 25, 465, and 587 through the firewall.
- Whitelist the email server IP address in security filters.
- Ensure DNS records (MX, SPF, DKIM, and DMARC) are correctly configured to prevent email blocking.
3. Fixing SSL/TLS Certificates
- Ensure the certificate matches the domain name:
- The CN or SAN should match the email domain (e.g., mail.example.com).
- Renew expired certificates before they cause disruptions.
- Use a trusted Certificate Authority (CA) to issue SSL/TLS certificates.
Check certificate installation using:
openssl s_client -connect mail.example.com:443 -showcerts
Configuring and Correcting SMTP Settings with Warmy
For seamless email deliverability and security, Warmy provides SMTP configuration support. You can check how to properly set up SMTP with Warmy using this knowledge base.
Besides on helping you configure and correct SMTP settings Warmy offers different resources that can help you avoid other SMTP errors, such as:
Free SPF Record Generators
Misconfiguration of SPF records can result in email authentication failures, increasing your chances of your emails getting rejected or flagged messages and potential security vulnerabilities.
However, SPF alone does not directly impact SMTP STARTTLS errors, which are primarily related to TLS encryption and server communication rather than sender authentication.
Warmy’s free SPF Record Generator can help specify which mail servers have the authority to send emails to a specific domain. It helps develop a more streamlined creation process of SPF records, and overall development of email security.
Free DMARC Record Generator
The DMARC Record Generator helps domain owners specify how their emails should be authenticated and define the actions to take if authentication fails.
While DMARC primarily focuses on email authentication, maintaining proper policies alongside secure SMTP STARTTLS configurations helps enhance overall email security and prevent delivery issues.
Free Email Deliverability Test
This will help email users conduct proper evaluation and develop proper fixes to avoid most common and complex errors that may affect their deliverability. The free email deliverability test also comes with IP or domain verification. Ensure that your mail server has a correct and healthy setting by verifying up to 100 DNS testing per month.
Incorporating this strategy with the automated email warmups can improve the reputation of your domain and potentially exclude your domain from being blacklisted.
Domain verification, alongside warmups will help improve the reputation of your domains and exclude you from being blacklisted.
AI-Driven Email Warm-Up
Our research study concluded that gradual volume increase is the warm up strategy that offers the best impact in email deliverability.
New and inactive domains often suffer the consequences of having no established reputation for most ISPs, which makes it difficult for some of their messages to hit the inbox.
Our automated email warm-up simulates human-like interactions such as sending, receiving, marking emails as important, and preventing emails from being flagged as spam.
This is a vital strategy for developing a new email user reputation and guarantee that newly created domains obtain credibility with various email providers.
Domain Health Hub
It helps evaluate every statistical data at a domain level and is a perfect tool for a comprehensive DNS status check, and ensures the ideal domain health that will help you avoid other potential issues.
- Domain Health Score
You can confirm the status of your deliverability with the instant domain health score, metrics such as inbox placement and DNS authentication contributes with identifying any misconfigurations or security restrictions
- Track Spam Rates, Inbox Placements and Deliverability Trends
Tracking spam rates, inbox placements and deliverability trends weekly and monthly are possible with the clear warm-up performance insights and One Click Deep Insights.
The one-click deep insights will allow you to click on any domain to access detailed health metrics and performance reports.
- Validate and Troubleshoot Essential DNS records
Minor issues, misconfigurations, and insufficient security settings can lead to various email errors. While DNS misconfigurations are not a direct cause of SMTP STARTTLS errors, maintaining properly configured DNS records helps prevent authentication failures and other deliverability issues that could indirectly impact secure email transmission.
DNS status checks validate and troubleshoot essential DNS records for seamless email deliverability. It can validate and troubleshoot the following:
- SPF
- DKIM
- DMARC
- rDNS
- MX and A Records
Conclusion & Next Steps
SMTP STARTTLS errors can compromise email security and prevent successful email transmission. By understanding the causes—such as certificate mismatches, misconfigurations, and port restrictions—and applying the fixes outlined in this article, you can ensure secure and reliable email communication.
Key Takeaways
- STARTTLS ensures email encryption and protection from MITM attacks.
- Certificate mismatches, misconfigurations, and port blocks are the most common reasons for STARTTLS failures.
- Correcting SMTP settings, firewall configurations, and certificate validity can resolve most issues.
- Warmy provides an automated solution to optimize SMTP settings and improve email deliverability.
Next Steps
- Check your email server settings for STARTTLS support.
- Verify SSL certificates and renew if needed.
- Use Warmy’s tools to diagnose and improve email deliverability.
By implementing these solutions, you can prevent SMTP STARTTLS errors and maintain a secure email connection.
Sign up for the 7-day free trial, or book a demo to improve your email deliverability today!
