Outbound mail is backing up on the mail server and is not accepted by SMG.
When connecting to port 25 of the outbound mail IP on SMG, the response is "421 4.4.0 Unable to connect to DNS - try again later" instead of the usual SMTP banner. The MTA log includes many entries that state "SMS_AUDITLOGGER-02520: Domain container is NULL."
The hostname in the Outbound Mail Acceptance list must be resolved to an IP address each time a server connects to deliver mail. If the DNS server is inaccessible or too slow to respond, SMG will be unable to resolve the hostname and will reject all connection attempts.
Change the hostname to an IP address or a CIDR range. While a hostname will work here, DNS issues can result in all mail being blocked.
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