Calculating email attachment file sizes

There may be a difference between the reported size of an email attachment file and the actual size of the file.

This is because Sophos Email calculates the file size of email attachments using the email's MIME encoding, not the raw files.

Emails are encoded using MIME to preserve their content and format while being transferred over the internet. Because MIME uses Base64 to encode binary file attachments, there can be as much as a 37% increase in size compared to an attachment's raw version. For example, a binary file attachment whose raw file size is 20MB can exceed 28MB in Base64 encoding.

Email data loss prevention (DLP) policies

You need to take account of this when using attachment file size rules in Email DLP policies. These rules use the MIME-encoded size. We don't use the size of the raw files. You should increase your attachment size limits. See Create an email data loss prevention rule.

Reports

The size of attachments you see in reports such as Message History use the MIME-encoded size. We don't use the size of the raw files. See Message History Report.