Update 11/13/2023: We added new information on how to know when this change is released to your organization.
Today, we are announcing an update to our requirements for SMTP relay through Exchange ...
Updated Feb 22, 2024
Version 19.0
Blog Post
Saras870 are you saying that the goal in question is the ability to relay email from one Exchange on-prem. org, out/through several different EXO tenants? Assuming I'm that case you'd be selecting which tenant based on some criteria.
Your #1 was worded as multiple EXO tenants sending TO your server, which is opposite direction to this whole topic. Just making sure which direction you're thinking about.
To add accepted domain in EXO means to verify it when adding via M365/Entra admin centers. So those limitations apply re: adding to multiple tenants or not ( and match what Carolyn_Liu answered too).
But back to if the answer to my question is yes, assuming you have some domain(s) added to each tenant, you could have 1 additional cert in Exchange on-prem. per tenant, with a sub/child domain of the corresponding domain for that tenant in either the subject or san. These additional certs would be assigned the SMTP service and specified in dedicated Send Connectors' TlsCertificateName.
This seems rather busy and complex yet totally doable I think. And now I just double checked multi-tenant hybrid blog posts and see this is covered officially here: https://support.microsoft.com/en-us/topic/september-2020-update-to-exchange-hybrid-configuration-wizard-54278b2b-ea62-31e4-8be6-3b16dfa1b9b3
So there you have it, if doing multi-tenant hybrid, HCW had you covered. If not, I think still doable, manually with certs and Send Connectors.