Outlook 2013 error 0x800CCC78, cannot send emails

[David Lee]

Hello, I have been sending emails without any trouble for months on Outlook 2013, Windows 8.

Today, while sending an email to a number of addresses, the thing sent one email and then just started bouncing all the others.
The (non-)explanation given was 'Error 0x800CCC78. Cannot verify the email in your account properties. Server responded: 535 service is unavailable.'

As you may appreciate, this is next to totally meaningless to me. I pasted the error message into a search line, and got told to alter my outgoing message settings. I have never done anything with them, so they haven't been altered - certainly not in the middle of sending emails.
I had a look at the settings, and couldn't make any sense of them at all. So in ways I cannot grasp, I seem to have broken Outlook.

Please advise!
David Lee