User Agent: Mozilla/5.0 (Windows NT 6.1; rv:46.0) Gecko/20100101 Firefox/46.0 Build ID: 20160502172042 Steps to reproduce: Compose message to a recipient with an @aol.com address. Note: Encoding choice is Unicode; no attachments, just plain text. Delivery format set to 'Auto-detect'. Running Thunderbird 45.0 on Windows 7 Professional with full Admin rights. Actual results: Message bounces with 'undeliverable' message that states: ' The message contains a content type that is not supported <exchange1.[redacted].local #5.6.1 smtp;554 5.6.1 Body type not supported by Remote Host>' Expected results: Message should have gone through and been delivered as always before.
Issue was resolved, thanks to a Google search and a helpful solution posted by Rob van der Woude at http://www.robvanderwoude.com/tweaks.php#FixApps Problem was that the update to Thunderbird 45.0 reset the 'mail.strictly_mime' config command to false instead of true, causing mail to many recipients bounce.