winsock not connecting

[donW]

I have an VB6 email send program which calls up DUN (using catylsis.Dialer) to connect and
then uses Winsock to Send a email.
It works fine with 3 ISP's, but not with our major ISP!
It times out on the Winsock.Connect, with no reply from the ISP.
(The proper login connection has been made by the DUN, since if I try all sorts of changes to winsock.remotehost I get errors from the ISP.)
I have winsock:
.LocalPort = 0
.Protocal = sckTCPProtocol
.RemoteHost = xtra.co.nz
.RemotePort = 25
.Connect

Any ideas why it wont connect with only this one ISP?

[JoshT]

Because of SPAM and stuff, ISPs can be funny with how you can connect to mail servers. My dial-up ISP at home will not let me connect to any other SMTP server besides their own for some reason. I know of another ISP that will not let someone send mail through it without it coming from one of their own IP addresses.