hello, I found this information via www.support.xerox.com- I hope you find it useful.
Solution : Description of Maximum E-mail Message Size
When setting up e-mail and requesting to split into fragments, the printer will send split e-mails in 7 MB or 3.5 MB sizes rather than the maximum 10 MB requested.
The size of attachments cannot be used to determine the maximum message size. SMTP requires attachments to be encoded for transmission; in our case via Base64.
If a network trace is performed, the displayed amount of data required to be sent exceeds the size of the actual attachment by approximately 30%. This is because the attachment is encoded. The encoding includes headers, message body, and such.
The encoded attachment is what the maximum message size and number of fragments is based on; the total data transmitted not the actual attachment size. Any increase in the amount of data being sent (such as when encryption is enabled) will cause more, and smaller fragments.