Improve SMTP logging and retry

We are now using https://github.com/go-gomail/gomail to communicate with a remote SMTP server directly.
Ben suggested we can have a local SMTP server to handle logs, retry, etc.

Start by locating the Go code that uses github.com/go-gomail/gomail to communicate with the remote SMTP server. Clarify the intended local SMTP server, logging, and retry behavior, then define completion as those responsibilities being implemented and covered by appropriate tests.

Written by the indexing model from the issue text.