40 Production cloud users existing in O365 tenant on domain_a.com
domain_a.com MX points to EOP and specific transport rule catches email from domain_a.com senders and forces back out EOP instead of going on prem (per requirement)
On Prem 2013/2016 environment with domain_b.com
MX for domain_b.com points on prem
HCW executed successfully for domain_b.com
Requirement for centralized mail transport
When both Hybrid connectors are enabled in O365, email sent to user@domain_a.com hits the transport rules in O365 then gets forwarded to Hybrid Connector to push all mail to on prem. Mail never reaches cloud mailbox of user@domain_a.com. If I disable the Hybrid Connector to on prem it works fine.
I would have thought incoming mail to O365 for domain_a.com would have went through transport rules, then looked for mailbox, then forward to connector, but this does not seem to be the case. Is there any way to properly incorporate the existing cloud only mailboxes & users (users not sync with AAD) into this Hybrid? MS told me no.
OP: What you have described you cannot do. Incoming email hits mail flow rules and is routed onprem. Remove this mail flow rule and it will hit the mailboxes first. Ensure domain-a in EXO is listed as "internal relay" as the accepted domain value and then it will automatically route emails to onprem domain-a users. You cannot, with centralized mailflow, then route the EXO users out to internet and everything else via onprem unless.you create own connectors and not HCW connectors, but that is going to be tricky