If you’re seeing a 421 4.2.1 ‘Service not available’ message with a note that messages expire after about 840 minutes, it usually means that Tiscali’s mail servers are temporarily rejecting your messages. This is often part of a spam prevention method called greylisting. Greylisting works by temporarily rejecting emails from unknown or untrusted mail servers, which forces legitimate senders to retry and helps block spam.
When using Amazon SES, these rejected messages are automatically retried. If the problem is greylisting, Tiscali might accept the email during one of these retries. However, in your situation, the messages are expiring without getting accepted, indicating there might be a different issue.
This pattern suggests that Tiscali might be consistently delaying or rejecting emails from the shared IP addresses you’re using to send emails. Since this happens repeatedly over months and affects multiple recipients at the same domain, there could be a trust or reputation issue between your sending IPs and Tiscali’s mail systems.
To troubleshoot, start by reaching out to Tiscali or asking your recipients to check with their ISP. Confirm that email addresses are correct and ask if there are known problems delivering to Tiscali. It’s also helpful to see if any emails did arrive but ended up in spam folders.
Since your domain (fidelisapp.com) is verified with DKIM, SPF, and DMARC, and your SES account is in good standing, the problem is likely on Tiscali’s side, especially how they handle shared IP addresses.
If you have a paid AWS Support Plan, consider opening a support case. Provide details like the affected email addresses, message IDs or request IDs from your email logs, and the times when delivery failed. AWS Support can look into the shared IP pool and see if there’s a specific issue with Tiscali, offering guidance or escalating the issue if needed.
To help Tiscali with their investigation, gather and share information such as your sending domain (fidelisapp.com), the MAIL FROM domain used, sample email headers from failed attempts, the IP addresses involved (found in headers or bounce messages), and the times when issues occurred. This info will help them identify why your messages are being deferred and how to resolve the problem.