Today I have sent 2 messages to myself from 2 different servers. After I got nothing, I went to each server's mqueue folder and noticed error queue files stamped with dates of 7 hours after the message was sent.
The errors were the following:
Server 1:
read error from gourmet.spamgourmet.com.
Server 2:
Connection timed out with gourmet.spamgourmet.com.
(BTW, I used Neverbox in both).
Update:
Although the read error sounded more scary to me, that message has just arrived (which means 7 hours late)!
Update 2:
The message with the time out has arrived like 12 hours after I wrote this message (which means like 19 hours late overall).
I hope the fact both showed up in the end means you just delay message and don't lose them.