The CiviMail reply and bounce tracking VERP addresses seem to include multiple + signs in the address.
Eg. donotreply+r+21+21+ae1dc2b1ec827788@ , donotreply+ being the localpart
Office 365 now supports plus addressing (https://www.microsoft.com/en-US/microsoft-365/roadmap?filters=&searchterms=59441), which works with a single + character in the address, but addresses with multiple + characters are rejected (unknown address).
Is there a way to change the generated string to not include the extra + signs?
Edit - Had missed a CiviMail setting. VERP separator character can be specified at /civicrm/admin/mail?reset=1
The breakdown of VERP address is detailed in an answer to another question: https://civicrm.stackexchange.com/a/2327/2266