Retry about mailmerge HOT 2 OPEN

@awdeorio you said in #103:

I agree that reliable email delivery would be best implemented by starting with rendering all the templates into a folder.
In summary, I think that writing output to a folder is a niche use with a reasonable work-around. I don't think it makes sense as a stand-alone feature. I do think it makes sense as part of the bigger picture of reliable delivery #36. I'm goingg to close this issue in favor of continuing the discussion under issue #36.

Could you sketch out your thoughts on this please? I was planning on simply feeding all the generated eml files to a script that works with an individual message and either gets it accepted by the SMTP server or returns a non-zero error when giving up:

ls /path/to/*.eml | xargs -I{} bash -c "send.sh {} && mv {} {}.sent"

This would alert me when any of the messages failed to send, and would fix itself later in case of an intermittent error. A bad eml would require a manual intervention (removing it from the queue and possibly generating a fixed version).

from mailmerge.

I'm thinking something like this

1. Create a directory RUNDIR
2. Render all messages to individual plain text files in RUNDIR
3. Send messages one at a time
   a. If success, move on the next message
   b. If failure, wait and then try again

Design decisions

• Should we delete messages that have been sent successfully?
• Where should RUNDIR be located?
• Which specific SMTP exceptions should trigger a retry?
• How long to wait between attempts?
• Where to configure the delay between attempts and max retries?
• What happens if mailmerge exits unsuccessfully for any reason? E.g., user hits Control-C, an exception occurs, an error in a database row. Delete RUNDIR and start over next time mailmerge is run? Start where you left off?
• What happens if template or database changes in the middle of a retry? Should we copy the template, database, config, and attachments into RUNDIR?

Testing

• Verify mailmerge --dry-run does not write any files
• Directory creation, even with different PWD
• Message render to files
• Max retry
• Delay between attempts
• Directory cleanup(?) on success
• Directory cleanup on error
• Resume behavior on second run after error (pending design decision)

from mailmerge.