Outlook Express failed to deliver emails

[#1]

We download all our emails using Outlook Express.  Recently two emails to my wife failed to be delivered by OE, although they had been received and saved by PN on Squirrel Mail.  Has anyone else had this problem, and how can it be prevented?  It's the first time this has happened in over two years as a PN customer.

[#2]

Do you have any rules set-up in OE which might have deleted then on arrival or put them somewhere else?
Do you have any local spam / anti-virus blocking s/w which may have got at the emails after OE downloaded them but before it stored them?
Many firewalls / AV software have options to scan email on arrival so check their logs / secrure areas to see if they have got them.

[#3]

I'm not much good with OE rules, Peter!  I once experimented with Outlook rules and got in such a mess (couldn't understand the terms etc) have now gone back to using OE.
I've got antivirus (NOD32 - excellent, though few seem to have heard of it) and also use ZoneAlarm Pro - no blocking problems with either so far.
One of the undelivered emails was from an AOL address, but we've had more AOL stuff since, without a problem. The other was an animated Christmas greeting sent via googlemail, but we get those sort of things all the time, again without problems.
Perhaps this was just a glitch and won't happen again.  It just means I'll have to make regular checks on the PN squirrel mail to see if anything else is kept back.
Anyway, thanks for your advice.