Problem description:
My messages won’t send.
Error code/error message:
“The following message could not be delivered to all recipients:”
What has been tried so far:
Sending messages. =\
Messenger version:
Windows Live Messenger 8.5
Windows version:
Windows 10 Pro 64x
Strange
Ah yes. That bug. 
Since this tends to happen with Escargot sometimes on a per person basis, I’d look into restarting Messenger’s process (as in actually end and restart the process) and logging back in to see if messaging won’t act up anymore.
MSN 7.5 is the only solution at the moment.