I've had the same problem since up-grading to Big Sur.
There isn't a full fix, as of yet - but if you do a search, within the mail app, it will show up, and you can then drag it to somewhere more accesible than a seemingly invisible bin!
Just for reference, the thread is here:-https://discussions.apple.com/thread/252046137
I take no credit for this, though - the kudos belongs to JohnDoe0007, on the other thread.
I didn't realise it was affecting more than a few users, but maybe there will be a quick fix os-update coming, as it appears to be more widespread than I'd thought earlier on today.
At least there's a way of getting the message back, I suppose!
But I'm also secretly hoping for a fix to just "arrive", as it's quite tricky to remeber not to delete (& lose it's location) and to drag it into the bin you'd have normally expected it to go to, of it's own accord!