Applies to all Email Security Gateways, running all versions of firmware.
Click on the "Help button." This will tell you how the device presents the size:
"Size - Message size in bytes. Note: The size of the attachment file as indicated in the Size column and in the Message Preview Pane is the encoded size, which is generally larger than the actual file size"
You may want to use a converter if you want the size in Megabytes or other units:
Other message size considerations:
Solution #1413 - What will my Email Security Gateway do when it blocks a message for attachment size?
http://en.wikipedia.org/wiki/Email_attachment - about MIME formats
Link to this page: