Validate sender validation setup

[rykov]

Sender validation is one of significant signals to spam filters. There are a number of standards (DKIM, SPF, DMARK, etc) that are all tedious to set up. This task is to add a validation command that will do a sanity check on the From address.

For this task, we'll take the domain of the From address and check it against the hostname in in the SMTP settings. The command will also allow specifying a sender hostname via CLI flags.