STARTTLS support and some other code cleanup

[NGTmeaty]

Description

The changes are here to send mail in an encrypted fashion with STARTTLS if possible. This increases mail deliverability. There are also some other code cleanup things in here.

Motivation and Context

This massively helps mail deliverability and assists in keeping emails slightly more private. STARTTLS has been adopted by every major email provider and is quite useful (and pretty easy to implement.)

How Has This Been Tested?

We've setup a node-sendmail server and have been using it to send hundreds of emails with the changes we made here perfectly fine. The code has worked perfectly, and we've yet to see any errors (that haven't been fixed) linked to it.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

@GreenPioneer thanks!

[GreenPioneer]

@NGTmeaty @JorgenVatle - really appreciate the time you guys took to give back to the project!!!