This sounds like it could usefully be used as an option to configure; something like: --with-exhaustive-dns-traversal WARNING: enabling this option could lead to an effective Denial of Service attack on your mail server if a remote domain with multiple mail servers was suffering a DoS attack of its own. Enabling this option will force courier to always try every listed mail server for a remote domain, every pass through the mail queue, until it finds one that works. For a domain with many MX records, this could take a very long time if all the servers are unreachable, and this would slow down all other mail in the mail queue as well. If you don't set this option, courier will sometimes skip some MX records for a domain that it detects has problems. On later passes through the mail queue, it will eventually try all the listed servers. While this will occasionally delay individual items of mail that could have been delivered quicker, it will speed up delivery for other mail in the queue.