Several months ago I did this for the first time. I found out my first therapist had retired, and I was kind of sad about it because I had in the back of my mind I might go back and see her again. I found her email online and wasn't sure if it was even a good way to contact her anymore, but I wrote her anyways.

I sent a short and sweet email, of course I wasn't sure if she would even get it so I might have spent more time if I knew she certainly would. I thanked her, told her what she meant to me, and gave her some updates on my life all of which was big picture stuff. The entire email was only 3-4 paragraphs I think, not long. I sent it and forgot about it.

Weeks later she did reply to me and said she didn't check that email frequently but she finally saw my email and was happy to hear from me. She gave me several short and sweet updates on her life as well, like she moved, that she was retired, etc. She also said that I had thanked her before too but she still appreciated it. It was a nice email and made me feel cared about, and happy that she remembered me and thought enough to reply. I replied to that one with another paragraph or two and a couple pictures. She replied back quicker this time, and said thanks for the pictures and wished me well. So in my experience it was pretty short and sweet. This was a therapist though that I had only had a relationship with for about a year, and only maybe 10 or so sessions since I was pretty wishy washy about needing therapy then.

Oh, and I did say I was with a new therapist and that I was in love with him, LOL. She said she figured as much and was happy for me .