Apple Mail outgoing message exceeds ISP size limit?

Question

A short e-mail with a 16MB attachment sent via Apple Mail was rejected because, at 21.9MB, it exceeded the 20MB limit from ISP Verizon.
Same e-mail and attachment sent from the same computer across the same ISP via G-mail had no problem.
Does Apple mail add 6MB of overhead to a message?
How can I fix that?

Answer 1

If the attachment is a picture or equivalent, it may be being displayed as such in the body of the email message. This could increase the size of the message as sent.
If that is the kind of attachment, try right-clicking (or Control-clicking) on it in the New Message window; from the contextual menu that appears, select "View as Icon".
Then test.