Outbound emails cannot be delivered to the destination and get stuck in the delivery queue.
Telnet test session to the delivery MX record doesn't work or get the reply code error "451 lowest numbered mx record points to local host" in the RCPT TO command of SMTP email sending process.
The destination MX record is not working properly. Verify that MX record in DNS name servers are pointing to the correct mail server.
In some cases, a restart of the DNS name server solved the issue. However, the administrator of the recipient domain should take action to solve the issue.
Also check the setting of the delivery bindings. Make sure it is set correctly or change it to auto.