Maintaining Authorized Outbound Addresses

Document created by user.oxriBaJeN4 Employee on Jun 22, 2016Last modified by user.oxriBaJeN4 Employee on Apr 2, 2019
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If you require additional IP addresses added, or IP addresses to be removed, contact Mimecast Support at

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


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. If your mail server(s) do not support this configuration, please contact Mimecast support.

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

Viewing Your Authorized Outbounds


To view your authorized outbounds:

  1. Log on 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.
  4. If you send email from a shared hosting provider (i.e. Office 365 or GSuite) a message will show at the top of the Authorized Outbounds page as follows: 'Your account is configured to process traffic from Office 365 / G Suite'. If you are using another 3rd party hosting service, these IPs will not be listed on your account. You'll need to contact Mimecast Support to ensure your account is provisioned appropriately for this traffic.

    Umbrella Accounts

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.

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”.

Confirming Your Outbound IP address


To confirm your outbound IP address:

  1. Log on to your mail server.
  2. Open a new browser window.
  3. Navigate to This displays the internet routable (public) address for your mail server.