Windows 8.1U (and FF, FWIW, as I don't think this has any relevance in my case; IE11 is on the machine)
Windows Live Mail 2012 connected to Outlook.com via DeltaSync
Everything was fine, even this morning, until suddenly I received this error 0x80048820 message and two messages refused to leave the Outbox, even when trying it manually. Plus I was wondering why there were no incoming messages for a while.
I followed the link given
Within that there are various suggestions/instructions including turning off the firewall. Push ("blow" on its own got edited out by, probably, an overenthusiastic and ignorant grammar-check robot...) me over with a feather...it worked.
And no, I did not operate Windows or any other Update today.
Let's hope it rights itself - perhaps with a computer restart - as one cannot operate with needing to turn off the firewall at frequent intervals...