Solution 1)
Make sure that you are logging in with the Steam account that was originally attached to that TruckersMP account.
Solution 2)
Change the e-mail address that is attached to your TruckersMP account .An example would be Yahoo!, Gmail or Hotmail.
Solution 3)
Make sure that the @ symbol is being entered correctly. You can copy the symbol in Notepad and then paste it when you are typing your e-mail address. You can also hold ALT and type 6, 4 using numpad to create the symbol.