I got this error message when trying to reply to an email via SquirrelMail:
Requested action aborted: error in processing
Server replied: 451 qq write error or disk full (#4.3.0)
I didn’t get any error when replying from Horde mail, but I’m not sure if the email message went or not.
I also sent myself an email from two other accounts about 5 minutes ago, and neither has arrived. Both of these accounts usually deliver the email within a matter of seconds.