TromboneChat

Is there a reason I can receive PM's, but when I try to send them they stay in my outbox, seemingly unsent to the intended recipients?

This feels like high school all over again, with the part where my PM's don't get responses.

They stay in the outbox until the recipient reads them.

As Harrison said, your PMs stay in your Outbox until the recipient has picked up and read them.
This might take a while (hours to days to ...)
Not all TromboneChat members log on or check their messages every day!

Yeah that’s a quirk of phpBB. I wish there was a way to change the terminology. Maybe I’ll submit a suggestion since we can’t be the only people who find the verbiage confusing

Sounds like a good candidate for the FAQ (that no one ever reads).