ISP's vary in how restrictive they are with the send mail functions.. Some will only allow you to send mail through their SMTP server when you are connected directly with them. Others allow 'send after recieve' which allows you to send mail only after you have recieved mail and therefore have given their server your login and Pwd and they can track back to the IP # that your recieve request came from and then allow the 'send mail' to go through... Others take the 'outlaw' approach and let you send mail without verification but those servers rapidly get flagged as spam sources and end up blacklisted...
After a year of arguing with my ISP, now when I am traveling I use a free SMTP server that is loaded on my notebook. Download is available at
Free SMTP server
http://www.softstack.com/freesmtp.html Extremely easy to set it up, even I did it first try.
It works great for me. I never have to hassle with changing settings, using web mail, etc.. My notebook now loads it's own SMTP server so my ISP's very restrictive policy RE their own SMTP server is no longer a problem.