tiggit continues to have difficulties sending mail… In version 1.0.02 I put loads of extra validation around the smtp protocol handler to be certain that messages have been successfully sent before removing them from the outbox.
For some email services – notably Yahoo! – this validation does not appear to work perfectly, resulting in messages being sent, but remaining in the outbox marked as failed.
The difficulty is knowing which messages have been sent, and which have actually failed. tiggit has a handy drop-copy feature that will automatically add a bcc address to all messages that it sends. This can be used to add yourself as a silent addressee to messages that are sent via tiggit, so that when you receive your own message you can delete it from the outbox.
Not really a solution I know, not even a messy work around, but until the bugs are fixed maybe a way of knowing whether a message has been sent.