Not strictly DIY but I know there are some knowledgeable people on
here that might be able to explain this in plain English so....

I've been trying to explain to the Mrs why emails that she sends or
receives sometimes seem to get delayed by hours, or even days. I've
always just accepted this, but in trying to explain why to her realise
that I don't really understand it.

Taking an example, she sends an email via a client (Thunderbird) on
her PC (in the UK), which uses POP/SMTP to connect to her email
provider Virginmedia, to a recipient in the UK who uses a client also
using POP/SMTP to connect to Hotmail. Her client is setup to send
emails immediately, the recipient has their client checking for new
emails every 30 mins.

What is the path that the email might take through the "internet
cloud" to the point that it arrives at the recipients PC, and what
might happen to cause delays in the email being received?