Forum Discussion
Exchange 2016 Mail Flow is Not Working
We had issues with updating to a latest Cumulative Update and messed up our EMS and some Web Config. It seems our Exchange Server is totally bricked. So, we decided to boot our Exchange Server from backup. The backup was dated September 2025. Unfortunately, after booting up the September 2025 backup, we noticed that the internal and external mail flow is not working (our Exchange 2016 is Exchange hybrid configured). The outgoing emails are stuck in Draft folder. The following troubleshooting steps have been done to no avail:
-Checked if the port 25 is open -> This port is opened
-Check the network settings if the Preferred DNS Address points to the correct DNS Server --> It points to the correct DNS Server
-Modified the DNS lookup under Exchange Admin Center > Servers > DNS Lookups > Internal DNS Lookups --> Added the IP Address of the DNS Server
-Modified the hosts file under System32 > drivers > etc --> Pointed the IP Address of the Exchange Server to the FQDN of the Exchange Server
Currently, are not sure of the next steps to do in order to fix the issue. Any advice?
3 Replies
After restoring an Exchange 2016 server from a 2025 backup, the server’s configuration may no longer match the current hybrid environment. Hybrid mail flow depends on connectors, certificates, DNS records, and transport settings that must line up with Exchange Online.
Even if port 25 and DNS resolve correctly, the following issues are typical after a rollback:
- The server may be using outdated connectors that no longer match Exchange Online security requirements.
- The TLS certificate used by the Send Connector may be expired or incorrect.
- The Hybrid Configuration wizard must be re-run because the original configuration stored in AD may no longer match the restored server.
- Autodiscover and internal/external URLs may be inconsistent with current hybrid settings.
- Transport Services may be running with stale configuration until the server is re-registered with the hybrid tenant.
The first step is to re-run the Hybrid Configuration Wizard to re-establish connectors, certificates, and transport settings. After that, verify the accepted domains, Send/Receive connectors, TLS certificate assignment, and the server’s Service Connection Point.
Restoring an Exchange server from an old backup often breaks hybrid mail flow because the restored state is no longer trusted by Exchange Online. Reapplying the hybrid config normally fixes the issue.
At this point, the most helpful next steps are to verify that all Exchange services are running, check that the Transport service starts without errors, confirm that your send/receive connectors still exist and haven’t lost their configuration, and make sure the server’s TLS certificate is valid and properly bound. It’s also important to review the event logs (Transport, MSExchangeFrontEndTransport, MSExchangeIS) for any configuration or authentication errors. If major components are corrupted, the safest and most reliable fix is usually to run a “setup /m:recoverserver” using the same Exchange build as the backup, which rebuilds the server roles without touching the databases. Given the age of the backup and hybrid dependencies, this recovery step often repairs the underlying issues faster than continuing to patch individual symptoms.
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