Use single-quotes to prevent interpolation when configuring Postfix

Unless you intend to interpolate, you should never use double-quotes. In
this instance, it was actually impactful -- `$myhostname` and
`$mydomain` were meant to be left as-is, referring to those Postfix
config parameters, but were accidentally interpolated and subsequently
generating nonsense (but not fatal) configuration.

Relay access denied fix

Reverse DNS does not match SMTP Banner

Can -> Can't

Resolve "Anyone can send mails from a local user as long as the recipient is another local user"

use apt-get to install packages

This fixes the output "WARNING: apt does not have a stable CLI interface. Use with caution in scripts."

Only call deploy hook when mail cert is renewed