This letter can be sent by an accountant to a client when representation is no longer possible.
Dear Mr./Ms. Clientname,
This letter is to inform you that I can no longer serve as your accountant. I am (retiring/cutting back on clients/in legal trouble/no longer willing to serve you).
If it's more convenient for you, I'd be happy to pass your file along to my colleague (name), who is a licensed (CPA/Enrolled Agent).
I also wanted to remind you that you'll need to attend to (quarterly taxes/IRS issue).
Thank you for the opportunity to be your accountant. I apologize for any inconvenience.
