#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