Email Size Limits Policies

Document created by user.oxriBaJeN4 Employee on Sep 12, 2015Last modified by user.oxriBaJeN4 Employee on Sep 26, 2016
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Email size blocking allows Administrators to set email size limits for both inbound and outbound emails. This applies to the full aggregate size of the email and any attachments. Any emails that exceed the specified size will be rejected with the reason "Message exceeds email size limit".

The default Email Size Limit for the legacy Gateway is 100 Mb for both inbound and outbound emails, whereas the default Email Size Limit for the latest Gateway is 200 Mb for both inbound and outbound emails.

It is important to maintain the same size restrictions on your internal mail server to ensure successful delivery of emails through Mimecast. For example, if an inbound limit on Mimecast is applied at 20 Mb, and the mail server is restricted to 10 Mb, Mimecast will accept the inbound email, but will not be able to deliver it to your infrastructure.


Before configuring Email Size Limits Policies, it is important to understand the individual components that make up an email, and to understand the difference between "transmission size" and "binary size". The binary size is the true size of an email component, excluding any form of encoding required by the mail server to transfer the component via SMTP. The binary size of the component including the necessary transmission encoding, makes up the transmission size.


When configuring an Email Size Limits Policy, the threshold must be set to the maximum transmission size you would like the Mimecast platform to accept. The threshold functions the same as the "Maximum message size" threshold on a Microsoft Exchange Server does. The transmission and content encoding can add significantly to the total size of the email. This means that an email with a 70 Mb attachment, can have an overall size larger than 100 Mb.


The Email Size Limits Policy overrules the Attachment Block on Size Policy and the Attachment Hold on Size Policy. So an Email Size Limits Policy restricting overall email size of 20 MB will take preference over an Attachment Block / Hold on Size Policy of 20 MB.


Many Administrators need to be able to manage the size of emails allowed in and out of their environment based on company policies, or to conserve storage in their internal environment. Using Email Size Limits policies, Administrators are able to enforce certain limits for both inbound and outbound emails.

What you need

  • An Administrator Console logon with access to the Services | Gateway | Policies menu item.


Creating a policy


To create a policy, follow the instructions in the Creating / Changing a Policy article, but using the following options:


Policy NarrativeProvide a description for the Policy to allow you to easily identify it in the future.
Maximum Email Size (KB)

Specify the maximum size of the email in kilobytes.

Definition required?