Are you doing anything with SendAs permissions that might be making it fail? Example: Epicor doesn’t have the rights to send out emails for/from john.doe@fakeemaildomain.com? If you’re able to send out test emails then at least the pipeline is solid. Ours was built out via SMTP so can’t speak to what the complications were with a direct 365 connection.
Only hit on EpicCare is linked below for Epicor mail settings…