Email size from the sender is different from the detected email size of Hosted Email Security (HES).
Sent email is in .msg format while the email received or quarantined in HES is in .eml format.
For example, the email size from the sender in .msg format is 20 MB in size. This email was deleted by HES because the file size detected is 27 MB.
Emails in .MSG format is in a binary format and the attached file would also be in a binary format.
Size limits in HES are based on, not on the original file size, but on the MIME-encoded format. MIME is the Internet standard that extends the format of email to support the following:
- Text in character sets other than ASCII
- Non-text attachments such as audio, video, images, application programs etc.
- Message bodies with multiple parts
- Header information in non-ASCII character sets
Since the .msg and its attachment is in binary format, they must be converted to Base64 encoding to be suitable for SMTP or email transmission. This encoding may add about 37% to the original file size, this means that an original 20 MB file could exceed a 25 MB file attachment limit.
HES checks the Base64 encoded file size sent through SMTP connection and use it to compare with size limits.