Freeserve is one of those crap ISPs who prevent people from changing the From: header.
This prevents the user from sending email with one From: header when sending a private email, and sending an email with a different From: header when e.g. writing to a business contact.
In this way, FS keep away the more heavy users. They probably also block the usual ports e.g. P2P etc.
However, the above is an entirely different issue to an ISP preventing you logging in to his SMTP server when connected via another ISP. That restriction is almost universal.