#5.1.0 Address rejected.

    "A problem occurred during the delivery of this message to this e-mail address. Try sending this message again. If the problem continues, please contact your helpdesk."

    A quick check with mxtoolbox.com shows the following:  

    Reverse DNS does not match SMTP Banner

    If your receive connector does not match your fully qualified domain name (FQDN), you might get rejected messages with the #5.1.0 Address rejected.

    Let's say your server's FQDN is:  mail.mydomain.com

    But when you telnet into your mail server port...

    telnet mail.mydomain.com 25

    ... your server responds with:  220 mail.mydomain.local

    You have a mismatch.

    Using the Exchange Management Shell, we need to gather some information:

    Get-ReceiveConnnector | fl name,banner

    The response should be:

    Name : Default Mail

    Name:  Client Mail

    Name:  Relay

    Continuing with the Exchange Management Shell, change the receive connector to match the FQDN with the following statement:

    Set-ReceiveConnector "Default Mail" -Banner "220 mail.mydomain.com"

    If you telnet back into your mail port, your server responds with:  220 mail.mydomain.com

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