Just thought I'd record on here some strange behaviour I've just had in the Mail and Calendar apps.

By adding in a Calendar appointment with certain text in the Description/Notes, I was able to stop Calendar and Mail working, with an error message that could mislead.

I tried to enter a Calendar appointment with some details of a travel booking. However it didn't seem to be synch-ing to the web version of my account on Outlook.com. The text in the notes was quite long and it seems to be that text which may have been the issue, either the length or something in the content.

Worse than that, if I tried to synch Mail, it just says '<my email address> is unavailable'

I also tried to enter the same appointment online using Outlook.com and it gave me an error:
"Server Validation Error
Calendar wasn't able to complete this request."

Again it seemed to be the text in the appointment that it didn't like.

It does seem that this rogue Calendar appointment was stopping the synch, and also stopping Mail from getting through, with an error that suggested my account was unavailable when it was this one Calendar appointment, not the account which was the problem.

Deleting the Calendar appointment meant that Mail started working again.

Curiously I did eventually succeed, by entering another Calendar appointment on Outlook.com at a different time and gradually pasting in more and more of the notes, bit by bit, and then moving it to the correct time.