'Sendmail' when using the Sendmail mail server.
Don't. The Sendmail.py module is misnamed - it's really a
command line based message handoff scheme as opposed to the SMTP scheme used
in SMTPDirect.py (the default). Sendmail.py has known security
holes and is provided as a proof-of-concept only4. If you are having problems
using SMTPDirect.py fix those instead of using Sendmail.py, or
you may open your system up to security exploits.