Try and avoid html mails (use text only).

If you must use html mails (nb test text first) validate the html to see if there are any broken tags etc.

Ideally your "sent from" email address should be the same domain as what your SMTP address processes (check MX record to see if your mail server is in there).

SMTP server ideally should support reverse MX look up.

Don't use known spamming techniques (google it).

Don't use blacklisted mail servers.

DON'T USE !MARKS !! OR CAPS.

Avoid obvious marketing  phrases such as "pass it on" or "Everything must go!" etc.

Avoid selling viagra (nb if you are actually selling viagra please send me some ).

Thanks...