What you are experiencing is normal. For a fuller explanation see
http://www.postcastserver.com/help/Port_25_Blocking.aspx
As a general rule, the owner of the SMTP server must match the
entity providing the Internet connection.
The available workarounds 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. I don't think
Charter does. Cable companies are very bad about accommodating travelers.
3. Substitute the SMTP server belonging to your away-from-home connection
(Knology). This is a bit tricky, but if you want to go that route, I can provide
more details.
4. If you frequently send from multiple locations, get a free Gmail account,
configure it for POP access, and use it for your sending chores.
Gmail uses port 465 for SMTP, which is not blocked like port 25 is.