Patch — Re-queue message after encountering a temporary delivery error.

[fs]

Currently, if an exception occurs while sending the mail to the mail hub (e.g. connection refused, mail server refuses recipients, ...) the mail is lost!

While a proper solution would require some more advanced queue management this ticket is about the 'easy cases' when the error is only temporary and can be resolved just by waiting for a short time.

The patch attached catches some specific exceptions, freezes the worker thread for a second and puts the current message back into the queue.

[amcgregor]

Patch applied as of r28. Additionally, the thread dies due to the exception because a stale SMTP connection will only lead to a string of exceptions until old age otherwise. It's much safer (and easier on the logs for temporary errors) to simply kill the thread.