There are various communication protocols that can be used for SOAP. Http, Ftp, SMTP, etc..
The question is when should each one be used?
I'll get the ball rolling.
SMTP is good when you would like to do some multicasting or when you want to make use of its store and forward capabilities. For example, the receiver may not be up and running but you don't care as you don't require an instant response. You would like the webservice to just retry.