My guess is your filter is to complex, if you can not look at it and decide if a particular email meets the criteria. Almost all issues with filters being complex relate to folks trying to manage spam with manual filters. I hope than is not you.

I am not sure what too complicated means. I have about 20 items either checking the FROM field or the BODY field of incoming emails. I would think that a particular email either meet or fails each of these tests and is only flagged as a problem if at least one of the criteria fails. What should I do to simplify the filter. Does it make any difference if there is one filter with 10 items and another with 10 items of one with 20 items? My individual rules are checking from single words to perhaps a 6 word phrase.

There is no way to establish exactly which condition triggered the filter, so if it is to complex for you to work out just by looking at the email and it's filter then it is to complex for you to diagnose. Hence my point about complexity.

Since, if the rule is "too complex", valuable emails may be unknowingly deleted, don't we need to know what "too complex" means so that we can safely use the system?

Many thanks.

you are the one writing the rule. If you are unable to debug the filter then clearly it is to complex. IT is not a hard and fast rule, but I try to keep things simple in my filters so I can workout what is going on. Perhaps deleting email based on words in the body is in itself a bad idea. Perhaps you need to be placing ten in the junk folder by marking them as junk so you can review them before they are deleted.