Allow unauthenticated SMTP email for relays

[mngrant-zz]

We have an internal SMTP server that doesn't require auth. We can also use our Google Apps relay without auth. It'd be nice to have a "requires authentication" box that can be checked/unchecked for each specific use case.

[remie]

:+1: Great suggestion!

[GSWemigo]

I also need this :)

[romgen]

The idea is good but maybe this should be auto detected if no auth info is entered. Please avoid adding checkboxes that complicate things.

[lrineau]

The authentication could be deactivated if settings.email.username is empty.