Improve SMTP logging and retry

[louischan-oursky]

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.

[chpapa]

Main thing is for logging and retry which is common to debug in production

[kiootic]

I think the suggest 'SMTP server' is just to have a message queue to handle resiliency. In general, every message sending/batch operations should have this queue (e.g. web-hook events), which is NYI.

[chpapa]

yes it was just my exp the local smtp server handle it too well and easy for debug on production so my rule of thumbs was always have a local smtp server to handle all these.

Technically you can also have a common libraries that do the logging and retry really well and handle every kind of external system calls.