Email is an important component of many web applications. Recently we had a chance to appreciate the importance of timely, reliable mail delivery sent from an application.
Our client’s web application initially had modest email requirements, including account password reset requests, order confirmations etc. When we enhanced their event registration system to include waitlists, with sms and email notifications to users when limited duration waitlist windows opened up, timely delivery became critical. If a user doesn’t receive their notification promptly, there is a chance they’ll miss their window to register.
The sms messaging has been working reliably, but we received reports of delays in receiving emails, prompting an investigation. Viewing the mail logs, we noticed that some users’ mail servers delayed accepting our messages due to “reputation”. Our mail server resends undelivered messages to receiving mail servers, and it sometimes took a day or more for the last to finally be delivered.
Our first response was to double-check and tweak the DNS records, ensuring SPF, DMARC, and DKIM records were configured correctly, to let receiving mail servers know that messages sent from our mail server were authentic. However, even after confirming the DNS records were highly compliant, and adding some tweaks, some servers continued to delay receipt.
Next, we found that our mail server’s IP was blacklisted. Blacklists are used by mail servers to rate the quality of other mail servers. Not all blacklists are equally authoritative, and out of about 100 lists, we were included in just a couple. One list indicated that the administrators would accept payment along with a submission to consider removing us. On a review of the mail servers who delayed accepting our messages, virtually all were tied to the same organization: perhaps indicating that the organization takes a wholesale approach to checking blacklists when evaluating incoming mail.
The IP address used by our mail delivery provider for our domain is shared with other clients. Perhaps one of those clients has what could be considered reputation issues. One solution would be to upgrade the mail plan to so we have a dedicated IP, but this would be overkill for our mail volume. We requested a change of IP address, which we were granted, and the delivery delays promptly fell away. For the moment, our email delivery rates are good. If we show up on a blacklist again, we may have to upgrade to a dedicated IP.