Thanks Joe for the reply.
I browse to Properties > Services > Email Settings > Defaults > Layout adjustment and found the setting you mentioned but it's already set to auto.
At the MFP, while at the Layout adjustment screen, when I insert paper into the ADF, the original size option automatically change to "Auto Detect" too.
But when I scan and received the pdf in an email, the pages is in letter size and the bottom part was cut off at the 11 inches.
Btw, if this make any different, the MFP have system software 2.0.105.01.007.
Thanks