Some of our users in our remote branches are getting a 550 5.7.1 Unable to relay. We have two Exchange sites. Site A hosts HQ’s mailboxes. Site B is the site that hosts the branches. Site B mostly services clients that are configured as SMTP and POP3. There are 4 servers in each site, 2 HUB/CAS servers and 2 mailbox servers.
Some users running in site B get 550 5.7.1 Unable to relay errors from time to time. So the only way I can get the mail to flow again is to configure one receive connector to “Exchange Server authentication” in site A and one in site B. The other remaining connector must then be configured to “Externally Secured”(See Pic 1). Only then does the mail flow externally . But this configuration does not work for long. After a while the errors are back. I have tried a few config’s. If I put all 4 servers receive connector to “Externally Secured I get the following error: Cannot achieve Exchange server authentication. “ Attempted failover to alternative host, but that did not succeed.” Either there are no alternate hosts, or delivery failed to all alternate hosts. The permissions group on all receive connectors (Client and Default) are all selected except Partners.
It seems like only users that have an SMTP and POP3 configuration are impacted. Web App users and Exchange online mode are not affected.