Office 365

Are you sure the email accounts are set up correctly?

Did you check that all dns info is pointing to the correct place? http://mxtoolbox.com/

How long has it been since you made the switch?

Here is what was done and checked:
- Migrated from Google to O365 with migration tool
- Added all domains to O365 Admin portal
- Email accounts, set up and spelled correctly in O365 Admin portal
- Domain verification has been made in the O365 Admin portal

Old info: 123@abc.com
New info: 123@def.com
 - Added record in Exchange properties through O365 Admin portal to reflect 123@abc.com as acceptable SMTP setting

Still receiving Unrouteable address
Remote Server returned '550 Unrouteable address'

I should preface, 123@abc.com is the same user as 123@def.com and 123@abc.com is not set up in the O365 portal.

Sorry to say, Office 365 is garbage! I switched to and from a few times! Not sure if cost is what you're looking at, but I like webhosting.net and also 123together.com.

Customer desire.  Never had any problems until today.
Just figured it out ...  Intermediate hosting company.
Thanks for pitching in

I've requested that this question be closed as follows:

Accepted answer: 0 points for teksolgreg's comment #a39598855
Assisted answer: 100 points for padas's comment #a39598772

for the following reason:

DNS was the culprit on this one.  Previous provider did not remove their DNS mail records.
The mxtoolbox is a great tool, thanks as this should be a go to tool for anyone troubleshooting mail flow.

This might be it
Old info: 123@abc.com
New info: 123@def.com

Did you set abc.com as a domain alias?

DNS reports illustrated solution