I forwarded an email message from my Mum's BT email address to my personal email address (running on my own server on a VPS)
The email came through 12 hours later. Checking my server logs shows the first delivery attempt was deferred (as expected) but the next delivery attempt did not occur until 12 hours later. Being as I am not the only person to use Greylisting on my mail server. I feel this is something you might want to look at as it would appear the initial retry time on your server is not ideal. I can provide headers and server logs to demonstrate the issue if you like.
I'm continually having these problems. Over the last six months I've had important emails disappearing for over 3 months! The log files on my mail server say that the email has been received, and greylisted. The email then disappears back into the BT system for up to 3 months, before a delivery attempt is retried. If you don't believe me, I can supply the log files. This doesn't relate to one btintenet.com address either. Every one of my customers with a btinternet address is treated the same, until they get delivered from a mail server than my system recognises as having been previously whitelisted. Something is seriously broken here.