Manage safe sender lists for bulk mailers
If you want to use safe sender lists, you should know that Exchange Online Protection (EOP) and Outlook handle processing differently. The service respects safe senders and domains by inspecting the RFC 5321.MailFrom address and the RFC 5322.From address, while Outlook adds the RFC 5322.From address to a user's safe sender list. (Note: The service inspects both the 5321.MailFrom address and 5322.From address for blocked senders and domains.)
The SMTP MAIL FROM address, otherwise known as the RFC 5321.MailFrom address, is the email address that's used to perform SPF checks, and if the mail can't be delivered, the path to which the bounced message is delivered. It's this email address that is placed into the Return-Path in the message headers by default, though it's possible for the sender to designate a different Return-Path address.
The From: address in the message headers, otherwise known as the RFC 5322.From address, is the email address that is displayed in the mail client such as Outlook.
Much of the time, the 5321.MailFrom and 5322.From addresses are the same. This is typical for person-to-person communication. However, when email is sent on behalf of someone else, the addresses are frequently different. This usually happens most often for bulk email messages.
For example, suppose that the airline Blue Yonder Airlines has contracted out Margie's Travel to send out its email advertising. You then get a message in your inbox from sender firstname.lastname@example.org. In this case, the 5321.MailFrom address is email@example.com, and firstname.lastname@example.org is the 5322.From address which is the one you see in Outlook. Because the service respects the RFC 5322.From address, to prevent this message from getting filtered, you can simply add the RFC 5322.From address as a safe sender in Outlook.