Follow these rules and your mail templates will most likely not be considered as spam:

1. Always wrap the content in …
2. Remove all empty lines
3. Remove all font size definitions, like font:11px/1.35em
4. Avoid spam words such as “bank” or “lending institution”

Writing HTML mail templates that are not considered as spam
You want or need help implementing this solution? Maybe you did not find what you were looking for? Contact me and I will assist you with whatever Magento problem troubles you!