MimeMail silently failing to send node create and node update notifications

Experiencing a hard-to-track notifications failure, but testing is pointing toward Mime Mail:

- Using standard Drupal "Mail" send method, all notifications are sent properly
- Using MimeMail send method, OG node create and node update notifications are NOT sent
- However, MimeMail does send comment notifications from those same nodes just fine
- With Messaging Debug active, all messages appear in the dblog, including the MimeMail node create and update events that aren't being sent
- No errors appear in the log during failure conditions

Something seems to be failing silently in the mimemail module, the mimemail related hooks with the notifications API or OG notifications module, or something of that nature. (The various possible relationships between these modules are quite complex and hard for me to track.)

Or maybe there's a problem with the MimeMail message templates for OG notifications, which I was working on around the time the problem started. Something there (such as a token) may be causing MimeMail to choke silently, but wouldn't that cause the messages to fail to appear in the debug dblog as well?

Or is it possibly something with MimeMail's CSS compressor? My message templates for all node and comment events were pasted in from the same source, with only minor changes to the text and tokens to indicate node events, groups, content types etc. (Can you tell I'm drawing at straws here?)

Relevant versions: Drupal 6.13, Mime Mail 6.x-1.x-dev (2009-Jun-08), Notifications 6.x-2.1, Messaging 6.x-2.1, Organic groups 6.x-2.0-rc3, Token 6.x-1.12. Cross-posted from thread "Notifications just are not sent" (http://drupal.org/node/468280)