I am leaning towards doing nothing.
Not mentioning it even to IM.
Just letting go and not engaging in any way.

I have a different way for you to look at it. It wasn't a letter, it was an email. What would you do? I am hoping you would say that you would delete it without reading it, or even better that you had your WH's email blocked, and better yet, that you had changed your email addy.

THROW IT AWAY.

I remember the time that my WH set a letter for taking him off of our joint account on the porch. I cried about it. I was upset by it. Then I asked what I should do. I was told, "Put it back where it was and pretend it was never there." That was 5.5 months ago. I threw it away yesterday. I forgot it was even there. It was under a bunch of stuff.

Good call, reading. There is nothing in that letter that you would want to read. Nothing.

For cryin' out loud!

Just got a call on my landline and saw on caller ID a name I was not familiar with and answered.

I said "Hello?"

heard WH's voice say "Hello' all choked up.

I said "Yes?" and hung up after saying it.

Dawns on me WH was at his therapist's office and most likely mentioned that I was not acknowledging his email and postal mail and therapist told him to call me on my landline. I only recognized the caller ID name in retrospect from insurance statements.

I suspect it is not to reconcile seeing as he could tell my IM that and most likely he is trying to force a conversation about other issues.

Nope.

All I am sayin'. Now, I know the name/number of the therapist and will be careful to answer calls from unknown callers in the future.

edited to add....now bet this therapist encourages him to try to communicate with me in person. No! No! Don't do that therapist. I will just have to be ready for that if it is the case.

Ugh.

Deep breath here.......................phew.............

Would it be possible to just have a rule where you DON'T answer any calls/texts/emails/pigeons/etc. from unknown senders?

If it's important, they'll leave a voicemail. Screen that and respond accordingly.

I do as a rule not answer unless it is a known caller that I want to speak with but I answered cause I was watching a toddler for someone who was taking a nap and I didn't want to let it ring and wake the child I had just rocked to sleep.
I thought I'd answer to stop the ringing and because the first name on the caller ID is that of a neighbor on the next block (and obviously the therapist) and in my haste, thought it was her.

Haste to answer to keep a child from being woken up + not recognizing the last name= ooops.

Set the phone ring tone and answer machine volume at the lowest setting and you won't wake nothing.