After an office move the SBS 2011 server has stopped sending and receiving external email messages. Internal messages are working.
Clients - SMTP mail from mail clients (on ports 587, 993) succeeds.
- nslookup type=mx server 184.108.40.206 (or 220.127.116.11) show correct MX records
- Telnet to server first will get an unrecognized command on a ehlo or helo, but then will succeed on 2nd try.
- ping to server has 35% loss with IP and DNS
Server - Email stuck in Exchange queue get error: 451 4.4.0 Unable to connect
- nslookup type=mx server 18.104.22.168 (or 22.214.171.124) show correct MX records, but receive some timeouts
I have added DNS forwarders for our ISP into DNS service
I have added External DNS Lookups servers into Exchange.
I have cleared DNS server / service cache.
I have flushed DNS via ipconfig
Send connectors are using DNS for MX and External DNS Lookup.
At a loss for resolution. Seems extremely odd.