This is exactly why I never called a therapist on the phone without scheduling a call and only used text for quick practical notes such as scheduling changes. All of my outside communication with them was via email. I do not consider email intrusive - it does not demand immediate/quick attention and they can also choose to ignore it and not respond or not even read. It took me many repetitions and elaborate explanations to "teach" both of my Ts that, just because I like to send emails, I absolutely do not expect response or acknowledgement. Especially just quick messages back saying they had received my email and nothing else - to me that was not only pointless but also annoying at times. In the end both of them picked this up but it was hard to make them understand. I did not even understand why at first but then, reading PC and how many clients get anxious about Ts not responding fast or even at all - I guess that expectation is what the Ts were used to more. I have many work colleagues and own clients emailing a lot and I personally never find that intrusive - I deal with them as I can and don't feel forced to react immediately to every contact even though I generally do try to be responsive as much as it's realistic. I do find repeated unsolicited phone calls intrusive though and, to some extent, text messages as well if someone imagines texts are the written forms of phone calls.

With therapists specifically and with paid services, I also think that it has to be discussed and mutually agreed upon how much and what type of outside of appointment communication is included in the fee. But once a professional says, for instance, that receiving emails don't cost extra and whatever level of responsiveness is free, it's only good work ethic to keep the agreement. No need to exceed the agreement unless they choose to and then it's their decision and time.

In my communications, if someone contacts me too much or with stuff that I consider beyond the scope of our contact, most typically I will say it directly but briefly and if it persists, I just do not engage. That's basically setting and maintaining my own boundary. I don't think anyone is responsible for others' boundaries though, that's not even possible to regulate and not a good idea to reprimand someone constantly. People basically learn from experience what works and what does not in every relationship - if they are not willing to learn, maybe it's a topic for therapy but only if the client is interested/receptive to discussing it. In general, I think it is much more effective to regulate/maintain own boundaries than trying to influence others'.