Outbound PN mail to @btinternet delayed

[#1]

Certain emails from me to a colleague who uses @btinternet.com broadband and presumably Yahoo’s mail infrastructure are being delayed by exactly 4 hours. This seems to happen to the second of two emails from the same sender (me) to the same addressee sent in very quick succession.
Has anyone else experienced this?

[#2]

Have you asked him to identify where the delay is happening

[#3]

Few details as yet.
He does, in fact, create the emails himself so to speak: he is testing a PN-based online ordering system I have set up and is the Shopkeeper (the person the orders go to). He noticed the delay (exactly 4 hours) when he test-ordered something himself. The Shopkeeper email is generated immediately before the customer confirmation email and use the same 'from' address.
I have, of course, tried it myself (as both Shopkeeper and customer) - both with PN addressees and with hotmail addressees, and both emails arrive together.
I currently point the finger at Yahoo whose email system is known to leave a little to be desired. I doubt if any tarballing PN may give to bulk outbound email hits home at the second of two very short emails to one addressee.

[#4]

I just got on here to report a similar thing. Except this happens everytime sending to a particular address -
The original message was received at Sat, 9 Feb 2013 22:22:43 GMT
from <removed>.plus.net [84.93.230.250]
Deferred: Connection timed out with [46.236.22.136]
Warning: message still undelivered after 4 hours
Will keep trying until message is 1 week old
inetnum:        46.236.0.0 - 46.236.63.255
netname:        UK-ASUK-20101221
descr:              DediPower Managed Hosting Limited
This company is not getting my emails at all.
I was going to say I don't think its a PlusNet issue, but wondering if anyone has any idea what goes on?
(I'll start a new thread if its a different issue to this one.)
[Moderator's note by Jim (Oldjim)  email address removed ]
EDIT/UPDATE -
sorry about the email address.
Checked this on phone with support (v helpful, btw) - not PlusNet prob, it appears to be at the other end. So need to contact DediPower.