Retry on mail-send failure

Right now if mail-sending ("you need to write your snippets, yo") gives a 500, we error out. We should retry, or at least not give a 500. I think once we get a 500, we stop sending mail to everyone remaining on our list, which is Not Good.

No file or test is named. Start by locating the mail-sending entry point and tracing how a 500 is handled while sending to the recipient list. Verify the behavior with a mail-send failure and confirm that remaining recipients are handled without the request ending in an unhandled 500.

Written by the indexing model from the issue text.