You are right and email has never been a guaranteed method of communication. There are just too many servers along the way that can screw up and cause emails to go missing. Unfortunately if email just disappears into the ether (it has been known to happen with many ISPs over many years so isn't a Plusnet specific peoblem) then there is little way to find out which server was screwy since there is no header trace to follow.

I suspect this could be one of the reasons Plusnet require users to monitor the postmaster accounts. These emails tend to only go through 1 or 2 Plusnet servers before reaching the users rather than some unknown ones located anywhere in the world.