Have you tried switching on "my server requires authentication" for outgoing mail - this will require you to enter the username and password in the settings.
This works for claranet over BT broadband but they have a specific relay server to cater for that. It might work with freeserve too..
hth
amofw
ps: But if it doesn't work it will be because Freeserve is not a BT Broadband partner as explained
here