Maintaining Authorized Outbound Addresses

Document created by user.oxriBaJeN4 Employee on Jun 22, 2016Last modified by user.oxriBaJeN4 Employee on Mar 27, 2017
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Maintaining your authorized outbound addresses can only be performed by Mimecast Support. If you require additional IP addresses added, or IP addresses to be removed, contact Mimecast Support (e.g. by emailing who will be able to help.

During the implementation process, we'll add at least one IP addresses to your authorized outbounds based on the information provided in the Request for Information (RFI). These IP addresses are the only ones that Mimecast will accept outbound emails from. This is a security feature that prevents mail servers which aren't customer owned, from using us as an open relay. You can have multiple authorized outbounds, but networks cannot be added. 

It may take up to two hours for a new authorized outbound to be propagated and displayed in the Administration Console.

If you are using dynamic IP addresses for outbound email, we'll not add any authorized outbounds to your account. Instead, you'll need to configure SMTP authentication for your SMTP connector / send connector to enforce authentication with us. This means that when sending outbound email, we'll only accept emails based on successful authentication.


Viewing Your Authorized Outbounds


To view your authorized outbounds:

  1. Log in to the Administration Console.
  2. Click on the Administration toolbar button. A menu drop down is displayed.
  3. Click on the Gateway | Authorized Outbounds menu item. A list of all configured IP addresses is displayed.


If emails are sent from your internal domain, but from an IP address that is not listed, Mimecast will temporarily fail the connection with the error code:

“Open relay not allowed”


An open relay is a mail server used by spammers to send emails, even though these messages are not originating from the internal environment. By default, we configure Block Sender Policies, to prevent any external address originating from your authorized outbound, from sending emails to another external address. In other words we only accept emails from addresses belonging to your configured internal domains.


Confirming Your Outbound IP address


To confirm your outbound IP address:

  1. Log on to your mail server.
  2. Open a browser.
  3. Navigate to


This displays the internet routable (public) address for your mail server.