Things that contribute to a spammy email
Here is a list of things that contribute to making up an email that looks spammy to spam filters. Some are pretty benign.
1. Subject ends in numbers excluding current years. Example subject: testing 1,2,3
2. Email address ending in a number. Example: firstname.lastname@example.org
3. Using a free email provider such as aol, yahoo, outlook, ...
4. SPF not set. SPF is designed to prevent sender forgeries. More about SPF.
5. DKIM not set. More about DKIM.
6. Too many links and not enough text.
7. Freemail in reply-to but not from.