When away from home, sending is often disabled by a port 25 block.
As a general rule, the owner of the SMTP server you are using must
be the same as the entity providing your Internet connection at that
moment. For more on this issue see
http://www.postcastserver.com/help/Port_25_Blocking.aspx
The available workarounds when sending away from home are as follows:
1. Use webmail for sending (via your browser).
2. Ask your home ISP if they have a port other than '25' for SMTP.
3. If you frequently send from multiple locations, get a free Gmail account,
configure it for POP access, set it up in Windows Mail, and use that for
your sending chores. Gmail uses port 465 for SMTP, which is not blocked
like port 25 is.