Office 365 Outbound Mail Deferrals

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exchange_online_banner_sm  For those Office 365 Exchange Online Administrators, who deal with day-to-day Email/Messaging management, you may find from time to time that outbound emails from users in your Office 365 tenant are not being delivered to outside (non-Office 365 recipients) recipients. There are many reasons for this, which could include:

  1. Incorrect Email Address
  2. Disabled/Removed Mailbox
  3. Downed Recipient Email Server
  4. 3rd Party Desktop Anti-Virus/Malware Software
  5. *** 3rd Party Mail Solutions, such as Positini, IronPort, etc

*** When 3rd party mail messaging solutions are in place, which interact with inbound/outbound mail items as they are sent/received between an on-premises mail server environment and Exchange Online, additional configurations are many times needed in order to properly work (i.e. scan and deliver mail messages). Many 3rd party mail messaging solutions have a configuration setting to “allow” certain IP addresses and/or Fully Qualified Domain Names (FQDN) to be entered, allowing or denying communication with these.

Note – If you are receiving Deferrals, shown in your Exchange Online Forefront Online Protection for Exchange (FOPE), this means that the receiving mail server has responded to an Exchange Online Mail Server, attempting to deliver a message, however the 3rd party device did not outright reject the message.  This directs EXO to queue the message and retry the message delivery at a later time (i.e. every ~5 minutes).  In this scenario, you should review your 3rd party mail management solution and determine IF it is set for Allow/Deny and if enabled, verify that all known Exchange Online IP addresses and/or FQDNs are defined, so the 3rd party mail management device can/will receive messages from the Exchange Online environment.

EXO IP & FQDN Namespaces

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