I'm running a Hybrid Exchange 2010 / Office 365 environment and am in the process of migrating our users from on-premise to O365.
On my premise Exchange 2010 server, I have some specific Send Connectors setup between my domain and partner domains.
If a mailbox <g class="gr_ gr_17 gr-alert gr_spell gr_run_anim ContextualSpelling ins-del multiReplace" data-gr-id="17" id="17">on premise</g> sends to this domain (defined in Send Connector) the message delivers fine. If the mailbox is on O365, the message fails to send with error "#550 5.7.1 RESOLVER.RST.AuthRequired; authentication required ##" I have added the SPF entries for protection.outlook.com to our public records, and that has populated out, however, the error remains.
The messages are making it from O365 to my premise Exchange, and it is my premise Exchange that is rejecting the message instead of sending it through the connector.
I've already worked with and MSFT in the O365 group. He was unable to help and suggested I try this group instead.