A 550 5.1.10 bounce means Microsoft 365 looked up the address you wrote to and found nobody there. Usually the address is wrong or the mailbox is gone, so fix the spelling or stop sending to it. Less often the recipient's forwarding points at a dead address, or someone forged your address on mail you never sent.
How to read the result
On Microsoft 365 the code arrives as 550 5.1.10 RESOLVER.ADR.RecipientNotFound, often after "Remote Server returned". The decoder reads that wording as Microsoft's and puts its meaning first.
Then open the message in your Sent folder and compare the address you wrote to with the one the bounce names:
Same address, and you sent it. The address is wrong or the mailbox is gone.
A different address. Your message arrived, and the recipient's forwarding sent it somewhere broken.
You never sent the message. It is backscatter, and there is nothing to fix.
Who fixes it
A typo is yours: fix it and send again, and in Outlook delete the stale Auto-Complete entry first. A broken forward belongs to the recipient, who checks their Inbox rules, or to their admin, who checks the mail flow rules.
Backscatter belongs to nobody. Microsoft's guide to 550 5.1.10 in Exchange Online says it is harmless and does not mean your account was broken into. SPF on your domain makes the next forgery harder.
One oddity: the IANA registry gives x.1.10 another meaning, an address at a domain that publishes a "null MX" record, defined in RFC 7505. Both readings end at a dead address, so the fix rarely changes.
What to do next
If the address was right, keep it out of every list you send from. In Oneprofile, a hard bounce stops email to that address across the workspace, while LinkedIn steps to the same person still run.
A broken forward will not show up in any check you run. Forward the bounce to the recipient so their admin can fix it.