I believe this may be a system-level (Windows) setting but I am not positive. I did a quick test from our latest public release and find SHA-256 being used to sign the messages.
Signature Information:
v= Version: 1
a= Algorithm: rsa-sha256
c= Method: simple/simple
d= Domain: smartermonitor.com
Kyle Kerst
Acting IT Manager
SmarterTools Inc.
www.smartertools.com