winsock not connecting

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?

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.