Consider the following scenario in Microsoft Office 365 dedicated:
You have a distribution group that contains external mail contacts on a server that is running Microsoft Exchange.
You send an email message to the distribution group.
The header of the email message does not specify a return path.
In this scenario, some members of the distribution group may not receive the email message. Additionally, you do not receive a non-delivery report (NDR).
This issue occurs because some email servers do not accept email messages that do not specify a return path. This behavior is a method of spam prevention. For example, consider the following example Simple Mail Transfer Protocol (SMTP) conversation:
Server: 220 smtp.contoso.com ESMTP Postfix
Client: HELLO relay.example.org
Server: 250 Hello relay.example.org, I am glad to meet you
Client: MAIL FROM: <>
Server: 250 Ok
Client: RCPT TO: firstname.lastname@example.org
In this example, the recipient server rejects the email message because the message does not have a return path.
Most frequently, the email message does not have a return path if both the ReportToManager and ReportToOriginator distribution group attributes are set to False, as follows: