My T has done this in the past few months although I have also been emailing more and longer although it started out exactly like yours. In my situation when T and I talked about it he explained that his life and his work are both much busier than they were over the winter. He wants me to be able to communicate with him by what ever means I am capable of right now while we work on being able to do face to face. When he was feeling like he HAD to read them and acknowledge them before session he was starting to feel negatively about all the emails I send. When he stopped and thought about where the feeling was coming from though it was because he resented feeling obligated to reply and pressured to read them so that he could write the reply he felt obligated to send... even though they were often 3-5 word replies. When we talked I said the replies felt good but I didn’t truly need them and I know he has been clear that if you NEED a reply you need to call him and leave a voicemail. Now that he knows I am not going to be hurt or angry about not getting a reply I have been getting more replies again. Still not every email or even every week, nothing near what it had been but it isn’t total silence any more.
My T has been up front with me from the first session that he sees boundaries as more fluid they may change with circumstances inside and/or outside of therapy but that we can always talk about them in session. He knows I like certainties and absolutes so he has reminded me that his boundaries are fluid often.