Update – I have contacted my ISP and they have told me there is a block in place as too many emails were sent per hour. They've told me the limit is 200 per hour and 500 per day. I've explained that I can't understand why the website would have sent as many emails as that in one hour.
I've checked some of my emails server behaviours and they show:
BurstSize = "200";
BurstTime = "1";
WaitTime = "1";
Do you think I need to change these parameters?