Hi,
I've recently stood up a Hybrid Deployment, and I've trudged through most issues. I think my last one is sending mail from on-prem to the cloud. I've migrated a test mailbox to the cloud, and when I try to send email to it, the message eventually bounced after a couple days of delays. The delay message I get is:
Remote Server at Cas01.domain.com (10.128.13.66) returned '450 4.7.0 Proxy session setup failed on Frontend with '451 4.4.0 Primary target IP address responded with: "451 5.7.3 STARTTLS is required to send mail." Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts. The last endpoint attempted was 207.46.163.215:25''
I checked the send connector, and the servers are set to EHLO as the expected hostname, pass the expected cert, and TLS is enforced. Is there something I can check to determine why and how this breaking down?