As somebody who goes through great lengths to preserve my anonymity and protect my digital footprint, allow me to give a few basic pointers:

First off, if you want the best anonymity and security possible via email, use what is called "PGP encryption" which stands for "pretty good protection" and use at least a 4096 bit RSA key and here's why: a key that big would take the worlds fastest supercomputers millions of years to crack. If you want to learn more, a Google search will net you the knowledge you need.

Of course, the only problem with this method would be both parties need to have a PGP key configured in order for it to be effective. I would strongly suggest any therapist who deals with confidential client information to have PGP for their emails, solid network security, and good encryption for their computers and what not. It would be worth trying to convince your T to do this.

If the PGP option fails, another suggestion would be to get an email account that puts a heavy emphasis on privacy (preferably an offshore one). Sfletter, safe-mail, MyKolab, Jumble, PrivateRelay, RunBox, or Tutanota are all solid examples. If you value privacy and security, NEVER use Google, Mircosoft, or Yahoo as an email provider under any circumstance because they have been known to track you, sell your information, and even give your personal data to the government. I don't know about you, but when it comes to my mental health, I don't want any government or corporate entity knowing about my mental health issues and I will go to any extreme to prevent that.

Finally, it is worth mentioning to secure whatever devices you log in on your email from. Get a solid antivirus and if you're extra paranoid, also get a decent firewall and connect to your email via a SOCKS5 proxy. Also, I would suggest switching your default search engine to Startpage because they're completely anonymous and use 256 bit SSL encryption (the same encryption used via online banking websites) when you search the web.

Hope I helped!