PoP emails: something odd happening - help please?

[#1]

Hi, been on plusnet years but its my first forum post so...
On 28th October, an email popped into my Outlook 2010 inbox but date 13 months ago (it was from Dabs which I regularly receive).
On closer inspection, it consists of a 13 month old Dabs email concatenated with a legitimate email downloaded onthe 28th.
(Current AV Plusnet McAfee - changed from bought McAfee on 19th September last.
How can that happen?
Is it the Plusnet McAfee AV I use?
Can McAfee 'hold on' to old emails, and then mash then up with a new one?
Can Oulook 'hold on' to old emails, and then mash then up with a new one?
Any ideas? - I'm stumped.

[#2]

I wouldn't like to warrant a guess without seeing the email source. Probably wise that you don't post it here, however feel free to PM me the details and I'll take a look when I next get the opportunity.