550 Relay error with T-mobile

[#1]

I get this error while trying to send emails through my co.uk account on my laptop using a t-mobile usb hdspa modem.
Everything works ok on my wireless connection at home, after a bit of investigation online I found that the error probably arrises because I am sending mail through a PN account but from a wireless mobile account. This I guess is to stop spamming or spoofing.
Do I need to change the SMTP settings for my email account or do I require some further settings from t-mobile?
Unfortunately T-mobile won't accept my details to log in to thier support forum even though the usb modem has a sim card and a number.
Any Help appreciated.

[#2]

Thank you for your input.

Quote

Try smtp.t-email.co.uk

Where do these settings go on the email account?

[#3]

outgoing or SMTP server

[#4]

Thank you m8, this is the answer.
I have set up two email accounts with the same settings other than the smtp relay, one called home and one away so that I can send email from either my lan or the usb modem.
Would have been nice to have just one account that could do either depending if there was a network connection but what the hey it works now anyway.
Thanks again Peter for saving me losing a weekend trawling through help files.