The general rule is that you must use the SMTP server of the ISP that you've connected to the internet with. You should therefore be using an SMTP server provided by T-Mobile.
There are two basic exceptions to this rule:
1) If your own ISP's SMTP server supports "SMTP Authentication", you sign in to send mail similarly to signing in to the POP3 server to receive mail, i.e. with a Username & Password. You can then use any internet connection.
2) You can send mail via the *recipient's* SMTP server. Therefore, you can send mail to a Tiscali address using Tiscali's SMTP server, you can send mail to an Orange address using the Orange SMTP server and so on. If your own ISP is Tiscali and you're using their SMTP server in your mobile settings, this is why you can only send to a Tiscali address.