Need to find certain disappeared mails

Question

I know that a version of this question has been posted before...I won't bore you with specifics, but i'll just say that those posts leave me unsatisfied for various reasons.
So. I can't help it. I have to ask. Please help me out if you can.
I need to reference my sent mails from end May-June 2006. I use Mail as a POP account. Going back to retrieve the mails I need, I find that anything before June 29th has diappeared. I don't think it's a gmail thing, as it also happened on my old mac before I used gmail. It didn't happen when I used Outlook though, so I think its a Mail thing.
I just want to know if and how to get them back. They're pretty important (in hindsight).
Many many thanks!

Answer 1

David,
thanks for your reply. I've been away for a bit so didn't see it until now.
The OmniOutliner thing:
I figured a knowledgeable person such as yourself would be more thoroughly familiar with the full range of programs that generally come on a Mac, whereas there are some less often used ones that I don't use and, thus, I not familiar with. You may have in fact answered me, since you asked all these questions about it. I asked if it was a legit program existing on my Mac. Either the answer is no, or it's rarely enough used that you don't know it either. I can regard it as questionable. (Of course I didn't install/register with the program, or I wouldn't have asked you what it was. That is, unless I completely forgot doing so, in which case I certainly wouldn't ask you to read my mind and know that over myself.)
I was concerned because I feared it was one of those things that occasiobally come up when I'm on the internet, where a message that looks like it's generated by the computer pops up and says I may have some "errors", do I want to install some software to remove them. It's a scam that I'm sure you are familiar with. I feared it might be one of those, in which case I need to know so I can shut it down as soon as possible and not let it run in the future should I see it again. No, I don't expect you to know everything, but I figured you'd either be familiar with it or you wouldn't. You're not, so...now I know.
As for the Mail rules:
Now that you have mentioned what type of rules to look at, I can check (again), focusing on those types of rules to ensure that there are no rules selected that might cause this behavior. I do in fact pay attention when I get the "apply rules" message and believe I manage it knowledgeably (of course I could be wrong on that).
Everyone says it must be something in the rules, but I don't see anything of the sort, so I was asking more specifically what to look for. I have looked at my rules several times before for things that might cause the disappearance of messages and have never found anything indicative of the behavior exhibited by Mail. Having triple checked now the things you specifically mention, I can tell you that there aren't even any questions in the preferences about what to do with Inbox mails. I don't know where else there would be Inbox rules (maybe someone such as yourself does, i dunno). There is a place for me to give rules about Junk Mail, and a place to give rules for Sent Mails. There is no category of options for Inbox Mails that I see.
MacBook Mac OS X (10.4.9) using Mail