Free/Busy issue between and two Exchange on Line organizations, one hybrid

Hello,

here is my scenario. I opened this question to the M365 support teams, but it has been closed withou being solved, so I'm trying to ask the same to the community.

• Contoso.com is a cloud only Exchange on line organization
• Fabrikam.com is a Hybrid Exchange on line organization with HCW classic full and and Exchange 2016 on-prems server. Of course, autodiscover references the on-prems Exchange server.
• There are a federation trusts and Organization relationships between Contoso.com <-> Fabrikam.com (EOL) and Contoso.com <-> Fabrikam.com (EOP)
• userC1 is a mailbox user in Contoso
• userF1 is a mailbox user in Fabrikam (EOP)
• userF2 is a mailbox user in Fabrikam (EOL)

This is the issue:

UserC1  is trying to get Free/busy informations of a userF1 calling the primary SMTP address userf1 <at>fabriam.com successfully

UserC1  is trying to get Free/busy informations of a userF2 calling the primary SMTP address userf2 <at>fabriam.com NOT successfully (in remote connectivity analyser I can see the redirect to the target address to domain fabrikam.mail.onmicrosoft.com

UserC1 is trying to get Free/busy informations of a userF2 calling the alias SMTP address userf2<at>fabrikam.mail.onmicrosoft.com successfully

I think I must have made some mistakes, because I think it is not possible that MS has not treat this scenario. May someone help me showing me how can I solve it?

Thank you,

Enrico