Email Size and Attachment Policies

Document created by user.oxriBaJeN4 Employee on Sep 20, 2015Last modified by user.oxriBaJeN4 Employee on Jan 11, 2016
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Mimecast allows you to manage the size of emails or attachments.  Many Administrators require the ability 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.

 

This article aims to explain how these policies work and offers best practice information on their configuration.  To optimally configure these 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.

 

The diagram below represents the components of an email containing 2 attachments:

attach_policies.png

1 = Attachment in binary size

2 = Attachment in binary size

3 = HTML body of the email in binary size

4= TXT body of the email in binary size
5 = Message headers in binary size

6 = Transmission encoding

7 = Full transmission size

When configuring size-based policies, consider whether to apply the binary or the transmission size limitations.  Typically, a policy used to limit email size will use the transmission size of the email, while attachment handling policies will use the binary size

Available Policies

There are 4 policies which allow control of email and attachment size:

  • Email Size Limits
  • Attachment Link on Size
  • Attachment Hold on Size
  • Attachment Block on Size

Email Size Limits

When configuring an Email Size Limits Policy, set the threshold 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.

 

By default, the Mimecast platform applies an Email Size Limit of 200 MB to both inbound and outbound emails.

Attachment Block on Size

The Attachment Block on Size Policy triggers on the cumulative binary size of the attachments in the email (the value of 1 + 2 in the graphical representation above) and has priority over the Attachment Link on Size policy.

 

When configuring an Attachment Block on Size policy, set the threshold to the maximum cumulative binary size of the attachments you would like the Mimecast platform to accept.  As the Email Size Limits policy overrules the Attachment Block on Size policy, please be aware that transmission encoding can considerably add to the overall size of the email, and therefore you should increase the threshold of the Email Size Limits policy accordingly.

Attachment Hold on Size

The Attachment Hold on Size Policy triggers on the cumulative binary size of the attachments in the email (the value of 1 + 2 in the graphical representation above) and has priority over the Attachment Link on Size policy.

 

When configuring an Attachment Hold on Size policy, set the threshold to the maximum cumulative binary size of the attachments you would like the Mimecast platform to accept.  As the Email Size Limits policy overrules the Attachment Hold on Size policy, please be aware that transmission encoding can considerably add to the overall size of the email, and therefore you should increase the threshold of the Email Size Limits policy accordingly.

Attachment Link on Size

The Attachment Link on Size Policy triggers on the cumulative binary size of the attachments in the email (the value of 1 + 2 in the graphical representation above).

 

When configuring an Attachment Link on Size Policy, the threshold should be set to the maximum cumulative binary size of the attachments you would like the Mimecast platform to accept, and replace with a Download Link.  As both the Email Size Limits policy and the Attachment Block on Size policy overrule the Attachment Link on Size policy, you must bear in mind that transmission encoding can considerably add to the overall size of the email (typically up to 35% more and sometimes as high as 50% depending on the number of attachments).  Mimecast therefore recommends increasing the threshold of the Email Size Limits policy and the Attachment Block on Size policy, to allow the attachment to be accepted and replaced with a download link.

Attachments

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