Also, +1 on using Litmus for testing rendering across different clients.
There are also other things to consider when designing email templates and email deliverability in general.
- Always use multi-part, with a nice text part. Not having a text part is a spam flag.
- Keep the ratios of images and links to text low. High ratios are spam flags. Also, most of the time images will not be displayed by default.
- Use common sense, avoiding exclamations and referencing spammy words (eg. buy now!).
- Avoid link shorteners (commonly used in phishing emails)
- Use Litmus to test against spam filters to see where common sense fails.
- mail-tester.com is another nice free tool for checking for spam flags.
- There are also some little things that can trip you up, like having malformed message-Ids where the domain in the message-ID does not match the sending domain.
For more thoughts, we have a best practices (http://documentation.mailgun.com/best_practices.html)
Edit: Also, PLEASE only send emails to people that have given you (like on your website) permission and you have validated the email address with a confirmation link (double opt-in) before sending subsequent emails. Always give recipients the conspicuous and easy ability to unsubscribe.