Anne wrote:
Recently I suddenly received a number of emails that were sent some
weeks previously (eg. sent Nov 20, received December 15). There
seemed to be no connection between these - they were from a variety
of different senders and different ISPs. What has caused this and
how do I know there are not others that have not yet arrived?

If this was a one time event and isn't happening any longer:

There probably aren'/t, but there could be some that have just disappeared.
This sounds like a serious backbone crash somewhere and an bunch of mail got
stuck on some server enroute to yours.