Vista - Can receive but not send/reply to email

I am using Windows Mail (Vista) on an HP desktop and laptop computer. My ISP
is Charter. I use Norton as my security software.

My account settings for Charter are:

"Servers" tab of Windows Mail - Incoming mail: mail.charter.net, Outgoing
mail: mail.charter.net

"Advanced" tab - Incoming mail: 110, Outgoing mail: 25

The "Connection" and "Security" tabs are "blank".

When I am at my home location, I have no problems with either the desktop or
laptop computer functioning properly. When I am on the road (I spend summers
away from my home location) and connect to my email account using another
provider (my summer home phone company via DSL), I can receive email but I am
not able to send or reply to email.

This same problem occurs when I use my laptop in motels and other public
places on other trips.

The complete error message I receive is:

An unknown error has occurred. Subject 'test', Account: 'mail.charter.net',
Server: 'smtp.charter.net', Protocol: SMTP, Server Response: '421 charter.net
connection refused from [207.183.168.76]', Port: 25, Secure(SSL): No, Server
Error: 421, Error Number: 0x800CCC67

Any thoughts oh learned ones?

Rick wrote:
Yes, the mail server used to send will usually only allow users
connected to it's own network to send mail. When you collect mail your
"Authorization" to do so comes from your username and password. Neither
username or password have any effect on sending - your authorization
comes for the fact that you are a member of Charter's network - if you
were not authorized you could not connect at all. If you think about
this rule it makes sense because if they provided a "Open" server that
did not need authorization every spammer would use it.

Charter probably can give you access to their server from external
networks, you will have to then use a username and password in both
directions, Or you could ask them about using their webmail.

If you want the simple solution then when away from home set your SMTP
server name to that of your summer DSL provider, you will be on their
network so will be authorized.