Email Undeliverable

If you receive a notification email back when trying to send mail to someone - typically the subject line will say "Undeliverable:" before the subject of the email you sent. The notification usually contains the information needed to determine the reason.

This is also true if someone is trying to send you a message and they tell you that it is undeliverable. Having them tell you some information in the Undeliverable message (also called an NDR or Non-Delivery Report) can let you know what's going on.

Here is what is usually included in the NDR:

  • Generating Server: If our servers generated the message this will be something at the domain (e.g. or MG2.local (e.g. EXHUB1.MG2.local).

    If it was someone else's server it will be listed. This is NOT an indication of where the issue is but simply gives general information about where the message stopped.

  • Recipient: Usually the recipient will be listed. This should look like a normal email address. If it doesn't then something is wrong with the way the email was addresses. See

    If it looks normal, double check the spelling of everything. Sometimes our eyes play tricks on us so this is important.

  • Error message and code: These will usually look like this and we cover the common ones below.

    Remote Server returned '< #4.4.7>'

  • Original message headers: This will be a ton of information that can be used to identify issues. Most of the time the error message is enough to determine the issue.

Common Error Messages and Codes:

#550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found ##

If generated by our servers this error is usually preceded by an odd looking string that starts with "IMCEAEX". This means that the internal (system) address used for a recipient has changed. This is most common when a mailbox is deleted and recreated. Outlook caches the internal address of users and when a user tries to reply to a message sent from before the mailbox was recreated. Outlook's AutoComplete function also populates the address in a new message from saved data that may have the old address.

In general this can be fixed by following this article -




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