How Spamhaus blocklisted my SMTP verifier — a server that never sent an email
In short: I'm building an email verification service that works via live SMTP probing. One day iCloud started rejecting my server with 550 ... rejected due to listing in Spamhaus — even though my server had never sent a single email. What follows is an investigation with a wrong hypothesis, a humbling config discovery, a bureaucratic ticket rejection, and an unexpectedly happy ending. With…
An email verification service developer encountered an issue when their SMTP verifier was blocked by Spamhaus, despite never sending any emails. The problem originated from a generic-domain HELO used during staging setups. When the service moved to production, the developer forgot to update the HELO in the configuration, leading to the listing.
Spamhaus does not accept public resolver queries, leading to an initial misinterpretation of the blocklist status. It was discovered that the HELO must match the reverse DNS record (rDNS) for the IP. Additionally, the developer learned that the sender address used in the probe must be accepted by their own mail provider. After fixing the HELO issue and confirming the match between the HELO and rDNS, the developer tried to submit a delisting request, only to be rejected due to improper authorization checks.
The rejection letter included valuable information: listings expire a few days after the last detection, meaning the issue could potentially be resolved without further action, provided the cause was truly gone.
Written by urgent.news from Dev.to's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
