There is a limit in the total message size (message + attachments) transmitted. The limit is 40Mb and is calculated like this:
If the value of (number of [rcpt to] * [message size]) exceeds the 40Mb the transmission of the message will be canceled.
This limit is to keep the mailserver performance. We did find many clients sending very big messages with very big attachments to a lot of recipients and this end up slowing down the mailserver effecting to the rest of the users in the server. If you need send a bigger file to someone, just upload the file in your website and send them the url so they can download it.
