Did you re-run the Hybrid Wizard after renewing the cert and associated mail flow with the new cert?
Hybrid mail flow issue: Outbound emails from on-premises mailbox to office 365 users is not working

We have currently renewed exchange server certificate, and assigned IIS, SMTP, IMAP services. Afterwards, on testing mail flow, we noticed that emails send from on-premises mailbox cannot reach office 365 mailbox or user. Emails send from office 365 can reach on-premises mailbox.
Services from odd certificates cannot be unassigned its greyed out.
What can we do to resolve this?
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Andy David - MVP 152.3K Reputation points MVP
2021-11-25T19:06:33.22+00:00 If you dont want to re-run it then ensure the new cert has the correct subject and issuer and that is set for the send connector:
$NewCert = Get-ExchangeCertificate -Thumbprint <thumbprint> $TLSCert = (‘<I>’+$cert.issuer+'<S>’+$cert.subject) Set-SendConnector -Identity “Hybrid Send Connector” -TLSCertificateName $TLSCert
If it still doesnt work, what is the NDR you get when sending or the error in the queues?