emails marked as spam but not spam

[#1]

For the second time this week I suddenly found on squirrelmail a email in the spam box which was certainly not spam as it was a reply to me from a email sent to someone in my address book.  Sorry I have not kept it to send headers.
I don't look every day in spam box on squirrelmail as not bothered about spam but I am bothered if I miss emails that I want to see because your system has suddenly decided it is spam!
There was nothing is the wording that could be construed as spam in either of emails.

[#2]

No spam filter is ever going to be 100% reliable and false positives will occur from time to time. Hopefully the trials we've started with the Critical Path appliances (we do intend on trialling other solutions) will increase the effectiveness of our filters.