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Mimecast Undeliverable - Unknown Address Error

Problem

Mimecast Undeliverable - Unknown Address Error. You get the message:

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The following message to < This e-mail address is being protected from spambots. You need JavaScript enabled to view it > was undeliverable.

The reason for the problem:

5.1.0 - Unknown address error 550-'Invalid Recipient - https://community.mimecast.com/docs/DOC-1369#550'

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Further more, looking at the TRACKING diagnostics, you see the "Rejection Information" states, "Failed Known address verification."

The issue is that the email address does exist in Exchange. What gives?

Solution

Well Mimecast has a few settings to receive email. This setting is on the domain/internal-directory level (administration > directories >internal-directories).

There are a few options. One is "Accept emails for known recipients only." Accordingly, each user that you want to receive email for must be added to Mimecast. The first time a user sends an email outbound via Mimecast a user will be created.

Since groups don't send email (typically), a Mimecast account is never added. So it's possible that there could be an email address in EXCHANGE that is not in Mimecast.

Fortunately, users can also be added to Mimecast through:

  • import (ie import a list)
  • manually
  • AD sync

If there are not a bunch of groups, it's probably easiest to just add the group email addresses manually.

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