I think that ultimatums can sometimes work well if they aim to address something the other party is not doing responsibly, not sticking to their promises or prior agreements in a situation where there is mutual interest. I would not use ultimatums before exhausting most other forms of communication and before assessing my own part in it and whether I do participate sufficiently and my expectations are realistic.

I really doubt that a client can truly push a T to do what they don't want though, especially outside of session. Are you talking with him about your ongoing frustrations around emaling? Maybe that's how the experience could be most useful? You have definitely been through a lot regarding this email topic, including a period when you said stopping the emailing empowered you. It does seem like an ongoing struggle. I personally don't see demanding that he email realistic with a therapist unless they state explicitly that a certain amount/form/whatever of emailing is included in the service you pay for. I think acting out is fine but expecting the T to react according to those wishes that go beyond what is formally offered is quite unlikely to achieve anything but further frustration. As far as I understand, people who use therapy to work with similar interpersonal expectations do so by discussing it with the T. I don't think it is bad to act out occasionally though, that is what provides the material to work with I believe.

I relate to the above. I communicated with my first therapist in ways I never do anywhere else unless I am in a similar very frustrating situation where I don't see a solution and people are manipulating me. It's always similar kinds of situations/people that can bring that out of me but it takes a lot. I always find it very insightful to honestly think about what these types of reactions represent and why the strong feelings. I think the standard explanation that it comes from some childhood lacks or mistreatments is only one possibility - one that Ts and therapy models often propose, but I think it can be many more and I would encourage everyone to think outside of the box if the typical explanations don't feel right, both just ourselves and when talking with a T. In this case, for example, why this emailing thing has been such an ongoing frustration for you.