Policies - Controlling Email Size & Attachments

This article contains information on configuring email and attachment size policies in Mimecast, including Email Size Limits, Attachment Link, Hold, and Block policies, and considerations for binary vs. transmission size to optimize storage and email handling. To optimally configure these policies, it is important 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 transmission size is the binary size including the transmission encoding.
• Message components. The diagram below represents a message's components containing two attachments.

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

Email Size and Attachment Policies

The following policies can be used to control email and attachment size:

• Email Size Limits: When configuring an Email Size Limits policy, set the threshold to the maximum transmission size you'd like Mimecast to accept. The threshold functions the same as the "Maximum Message Size" threshold on a Microsoft Exchange server. By default, Mimecast applies an Email Size Limit of 200MB to both inbound and outbound messages.
• Attachment Link on Size: An Attachment Link on Size policy triggers the cumulative binary size of a message's attachments (the value of 1 + 2 in the image above). When configuring an Attachment Link on Size policy, the threshold should be set to the maximum cumulative binary size of the attachments you'd like Mimecast to accept and replace with a download link
• Attachment Hold on Size: An Attachment Hold on Size Policy triggers the cumulative binary size of a message's attachments (the value of 1 + 2 in the image 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'd like Mimecast to accept.
• Attachment Block on Size: An Attachment Block on Size policy triggers the cumulative binary size of a message's attachments (the value of 1 + 2 in the image above) and has priority over the Attachment Link on Size policy. When configuring an Attachment Link on Size policy, set the threshold to the maximum cumulative binary size of the attachments you'd like Mimecast to accept.

Email Size Limits policies overrules the Attachment Block on Size and Attachment Hold on Size policies. Additionally, both the Email Size Limits and Block on Size policies overrule the Attachment Link on Size policy. Be aware that transmission encoding can add considerably to the overall size of a message, between 35% and 50% depending on the number of attachments. Therefore increase the threshold of the Email Size Limits and Attachment Block on Size policies accordingly.