Using Outlook for iOS for personal email and calendar management
We just upgraded an older device to iOS 16.3.1 and began to experience this and instances of sporadic "Cannot Send Mail | An error occurred while delivering this message." Other symptoms include being unable to load various items in the expected formatting and instead see it in a Base64 encoded format, etc.
We've tried deleting and re-adding the the Microsoft 365 account, forced logouts, password resets, etc. and while modern authentication appears to be working fine, not just one, but TWO brand new iPhone 14 Pros with iOS 16.3.1 exhibit the same symptoms for one particular user.
Each time I think it's fixed, I'll get a sporadic "Cannot Send Mail | An error occurred while delivering this message."
I am now going to try disabling Notes syncing to see if that works for us, thanks for the tip.
EDIT: Upon some testing, disabling Notes did not seem to do the trick for us, so I'm going to start by disabling syncing one by one to see if we can find a correlation. Next is Reminders (Tasks).