I’ve been getting a strange error when trying to send pictures within e-mail messages from my Android phone. I say a strange error because there’s literally one entry that comes back when you search for MessageDeliveryFailedException f5f0 — and no pointer at all as to what might have gone wrong. Just a non-delivery report popping into the Inbox on my phone:

     Technical details
     MessageDeliveryFailedException: Could not deliver the message [len=70, data=50005…C090005] sent at 8/25/2020 10:06:28 PM.Failure code: f5f0

I’ve got a reverse proxy with an application firewall and suspected that was the source of my problems. Mostly because errors caused within the Microsoft Exchange system are generally easy to find online. An oddball error is going to come from an oddball source. And I was right — my application proxy log shows an error each time I attempt to send one of the failed messages.

Edited /etc/httpd/conf.d/mod_security.conf and upped the SecRequestBodyNoFilesLimit. Once Apache HTTPD was restarted, I was able to send my messages without problem.