If you'd like to improve mail deliverability, we recommend adhering to the following best practices.

Unsubscribe header

When sending mass-mail or mail that goes out in large quantities, we strongly recommend adding List-Unsubscribe header. This will improve mail deliverability and will help provide your users more comfortable reading experience.

For more info about this header, please consult RFC, list-unsubscribe.com, and a couple of blog posts: one, two.

If you decide to include the header, please ensure the header is actually working as expected: the unsubscribe request goes through and is handled accordingly.