On Wed, 12 Feb 2014 10:56:54 +0000, Davidm wrote:

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

* Her client hands the email to one of VM's mail servers. You'll see that
happening at her end.
* It might well go through a couple of different servers inside VM, where
it'll get virus and spam filtered.
* VM's mail servers check DNS for the MX records telling it where the
mail servers are for the recipient domain. The various MX records have
priorities, telling the sending server which order to try them.
* VM's mail servers then try to hand the mail over to one of the the
highest priority. If it's unavailable, it'll try another. If it's
unavailable, it'll try another, and so on down the list. If none are
available, then the mail will be held and retried later.
* If the recipient email address is a personal domain that's then
forwarded on to the actual Hotmail box, then it'll get to the forwarding
server, which'll re-send it with the hotmail address wrapped around it.
* Once in Hotmail's systems, it'll be spam/virus filtered and passed on
internally to whichever server or cluster of servers hold that
recipient's mailbox. If it's a cluster, then it'll be replicated around
the individual servers.
* The recipient's client checks the Hotmail server(s), and new mail's
then delivered.

Have a read of the full headers for any mail she's received, and you'll
see the names and times of each step in the chain listed.

There's plenty of scope for delays and hiccups, especially if there's
that extra link involved. But, between large infrastructures like VM and
Hotmail, it _should_ be damn near instant.

and what might happen to cause delays in the email being received?

The most likely cause is the undeniable fact that Hotmail is run by
raging ****wits, tbh. Their spam filtering is a law unto itself, and
about as over-eager as a Spaniel puppy coming up to walkies time.