Really it's daft to block you from using port 25 to send email via your own server which requires authentication but that's what some WiFi ISPs seem to do.
Find a port other than 25 on which your SMTP server will play and use that, and hope that the WiFi ISPs don't spot people doing that and block that one too.
I'm not going to repeat what Mike said.
What's "really daft" is to run your own SMTP server when you really don't need to, and when you don't understand the reasons for anti-spam measures and how these can be legitimately bypassed if you need to.
Basically, what I'm trying to say is :
- If you're a private individual (or a one-man business) then there is really no need to run your own SMTP server. There are many other ways to achieve what you need without the headache (e.g. "smtp.com" and other similar services).
- If you're a business, then you probably ought to look at changing IT providers if they can't figure out a way to give you roaming email (or just moan loudly at your IT department in larger companies).