I have received an email and when I try to reply to it I get a failure notice saying that the email address does not exist.
presumably if the person emailed me then the email address must exist so why would I get this error?
Is there any way to find out what is going wrong?

I had this once, it was to do with their reply pop address which was wrong, when I looked at the properties it showed a 'root' tag, which was different to their normal address.  To get round it I didn't reply but sent a new email and it went ok.  I am not technical and this may not be your problem but it was how I got round it.

I had this too not long ago, I don't know any technical details why, but I too sent a new email instead of replying and had no trouble with that one getting through.

I once had this, and it was just because it was a web based email address, and they had gone over their allotted space.  

thanks. a new email also fails. I don't have a phone number for this person/ Will just have to wait and hope they try again