Several times recently, people have told me they sent me an email, only to have it rejected on the basis of its content. The messages in question are all perfectly ordinary, such as an invitation acceptance. Could it be that there are too many or too aggressive spam filters in my ISP's system or elsewhere, or is there some other possibility?

That is what I would think it is - the spam filter that your ISP is using.

But also, if you are using Ms Exchange, your administrator may have set up rules that are too strict.

Assuming you are using Outllok you can set up your own anti-spam rules (Tools|Rules Wizard).