There is a program called nbsmtp. It allows to specify which MTA you would like to use.
Download and compile program
http://nbsmtp.ferdyx.org/
Code:
cat /tmp/mail.txt | ./nbsmtp -f you@yourdot.com -h remote.relay.email.server.com -p 25 -u smtp-user -p smtp-passwd -M 1
or
Code:
./nbsmtp -f you@yourdot.com -h remote.relay.email.server.com -p 25 -u smtp-user -p smtp-passwd -M 1 < /tmp/mail.txt