How do HTTPS settings interfere with CiviMail?
It sounds like you have "http://" hard-coded into your URLs.  Check your BASE_URL in civicrm.settings.php (and search the whole file for "http://" while you're there), and check the Administer » System Settings » Resource URLs menu to ensure that your resource URL doesn't include "http://".

Finally, check if you have any extensions that have the text hook_civicrm_getAssetUrl, which suggests that they may be changing the Asset URL.