Add main verify config option.

[davidlehn]

Add some sort of main config verify control option. Use case is development config setup to use, say, localhost. If you have a SMTP server you may want to verify it works even if not in sending mode. But if yout don't have a server then the check will print out a potentially confusing error. One option is to override it to only verify if also sending, and provide a CLI override for that?