Do you have any idea when this happens? The log you mention only shows a single mailing attempt.
No - thats basically the reason why I'm asking this question here...
Could it be it only works from one of your accounts and not from another?
What do you exactly mean? I have 1 bot account only. This one exists for years and mailing worked fine in the past (I had written my own mailing routine/function until I discovered the one in the framework) at least until we had to change to use SGE and cronie on the toolserver (instead of crontab)... but I don't know if this is affected by toolserver changes (as far as I understand - it should NOT)
Have you tried sending an e-mail manually? (i.e. logging into your bot account and using Special:EmailUser, or whatever it's called). Does that work?
As mentioned in the past mailing from the bot and bot account worked and anyway it works even now, but just 50% of the times (roughly guessed)... In fact it worked the day before as visible from another log in [1]. (In the past I even had a "MailerBot" running - that had mailing as only job)
It is a somehow frustrating thing; if I had a clue what could be the reason I would add more debugging output... And DO more debugging in the bot - but as you can see this is a rare and alternating bug which makes tracking down hard...
Greetings
[1] http://toolserver.org/~drtrigon/DrTrigonBot/20110723.log