Brother MFC-8640D - Manual (Page 157)

7 - 42 USING YOUR MACHINE WITH A MACINTOSH ®
Scan to E-mail:
In the General tab
You can enter up to 30 characters in the Name for Custom field.
This name will appear as the button name.
Select the type of scan from the Scan Action field.
In the Settings tab
You can select E-mail Application, File Type, Resolution, Scan
Type, Document Size, Brightness and Contrast settings.