I'm fairly sure that my email could not be linked with my phone number.
Anyway, they way they filter through your text messages is by looking for keywords. So just be vague, say like "hey, I got the stuff" or something. And be sure not to say "This shit sure is da bomb!" Because then they will think you are a terrorist and come kick your door in.
Thing is though, you would know if someone was keeping tabs on you. I'm sure whatever it is you are doing is tiny and insignificant to them. Unless you are growing over 100 weed plants or selling fairly large amounts of heroin or something they don't give a fuck. I have only known a couple people in my life that have been big enough to get on their radar, and all of those people were very aware of the fact they were being watched.