New Exchange Server Message Size Limits
A note about an increase in the single message size limit on our email servers.
Please note that messages over 20 megabytes cannot be handled by the helpdesk ticketing system, so please do not send messages larger than 20 megabytes to computer support, as they will be deleted without notice to the sender or receiver.
The UC Davis tenant of Office365 has a per message limit of 150 MB. Messages over that size will be returned undelivered.
https://ucdavisit.service-now.com/servicehub/?id=ucd_kb_article&sysparm_article=KB0000556