I noticed this error today:
Jul 29 00:54:50 light imaplogin: malloc: Input/output error

Note helpful lack of pid, so I had to strace all my imapds to track it down and then grep through source code for calls to perror (random guesswork). And it’s caused by… wait for it… famd not running (see #294656). Of course!

No, really. WTF?!?! I’m switching to dovecot. Utter utter crack.

Update: Corrected the bug number. #308313 is the other problem I’ve been debugging today, the combination of the two leading me to believe my machine was rooted or had f🤬d hardware. I utterly hate both courier *and* proftpd, and will be switching to dovecot and vsftpd as soon as possible.