I will offer a different opinion than the previous posters.

I don't think it is necessary to lay out strict email/texting rules just in the beginning because most people won't abuse them anyway and for those who will start doing it, this could be an important symptom that would be relevant to observe, which would be impossible if the rules would be clearly set right from the start.

I also don't think that the T has to allow this kind of acting out indefinitely. The goal would be to see what it is and then try to redirect this stuff into session. With some people it can be enough to just talk about the issue, with some people it might be necessary to stop responding and with some people even that is not enough and it is justified to stop reading these texts altogether.

The main goal of a therapy is not to make the person feel good in the moment but to aim for better regulation and adaptation, greater autonomy etc in long term perspective. For that to happen, it must be possible to work with emotional material in session and if reading/responding to texts hinders this, then not engaging with these activities from the T's part is justified. He would be irresponsible if he wouldn't do it.

So, yes, you are stripped of your power to send angry texts but you have all the power to go into your session and say all these things in person.

Edited to add: you are even not stripped by your power to send angry texts but you cannot control whether another person choose to respond to them or even read them as Anne pointed out.