Mail keeps correcting "TM" to the superscript

How can I get my Mac Mail (and the computer in general) to stop autocorrecting "™" to the superscript - like it did right here? Sometimes it does the autocorrect and then shows me the option to "x" it but in Mail it is not doing that, and anyways I use a software program that is abbreviated to those two letters and I don't want to have to keep going back and changing it when referring to it in an email. It still does it even when I turn off the autocorrect.

Thanks, Chris