From MobileX for Sage CRM Help
When viewing an entity on your mobile device, if you wish to File an email, click on the plus icon
Then click on email icon.
A screen displaying the email, subject and message is there to be completed.
This email is automatically saved in your CRM database as a communication
Signature
If you created your email signature in CRM (My CRM -> Preferences -> "Create Email Signature" button) then when you create an email via MobileX it will put in your signature in the body of the email.