Add safeguards to script

Correct grammatical and phrasing errors

Make it work with Debian 12: spamassassin -> spamd

Adding pop3 support

This commits checks for /etc/default/spamassassin. If it exists, it's
passed through sed to modify the CRON variable as usual, and
spamassassin.service is enabled and restarted.

If /etc/default/spamassassin does not exist, but /etc/default/spamd
exists, we modify /etc/default/spamd instead, and restart and enable
spamd.service.

This has to be done because Debian 12 introduced this breaking change:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1020859

Apologies for my previous commit which introduced a new "README" file
specifically for my fork of the repository. I've modified my patch to
remove it, please accept this version of the patch instead.

Signed-off-by: Andrew Yu <andrew@andrewyu.org>

fix comment

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"