After adding a domain to O365 tenant, getting a bounce back when emailing from domain already in O365. So @domaina.com is already in O365 and we are planning to setup a Hybrid Exchange 2010 with domainb, so when domainb is added to O365 and a firstname.lastname@example.org emails him he gets a bounce back. We have not setup the Hybrid yet as the server still needs to be patched to SP3. I tried setting domainb to Internal Relay in O365 so it would look for the MX records but still get the same error. Do I need to setup a connector with the Exchange server?
Original Message Details
Created Date: 3/6/2018 7:32:24 PM
Sender Address: email@example.com
Recipient Address: firstname.lastname@example.org
Reported error: 550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient email@example.com not found by SMTP address lookup
DSN generated by: DM5PR19MB0954.namprd19.prod.outlook.com