IE6 won't send to some ISP's

When attempting to send e-mails to certain ISP's, such as Tesltra BigPond or Hotmail addresses, IE6 responds with error messages such as error 550, relaying denied etc. I can send to other e-mail users, the only problems detected are with the two mentioned.
I have another PC which gives the same error message, and it uses IE5.5..... is it something to do with the receiver's security settings not allowing acceptance of my mail? They can send to me OK.....

I know there are issues with IE6 security verging on the paranoid, but this is ridiculous.


Any clues ?

 

See if this helps:

Outlook/Outlook Express : Error 550 Messages