one of our coworkers is like "our customer is reporting that messages are disappearing sometimes, has been happening randomly for months, what gives, also where does this log come from"

4 hours later, after we've grabbed a release artifact, recursively unpacked it to grep for the log messages in question, referred to RFC 5322, threw around ncat directly to SMTP servers, dug around countless logs and configs, while coworkers were also doing similar things, we figured out that

our outbound postfix relays are rejecting RCPT TO: <-whatever@example.org> for obtuse unix reasons that are not applicable to this situation, and we can turn off this behavior with a config switch

and this is why they pay us the big bucks