I've received several emails over the past few months which say things like "This is a reminder that Invoice invoice number is now overdue. As per my Terms and Conditions, a late payment fee will apply if payment becomes overdue by 14 days after which a late fee applies."
There's no attachment for the unwitting person to open and no hyperlink for them to click on. The only thing that might happen is that they might harvest your address as an active one if you reply to say "what's all this about?".
The example I received today is particularly obvious as a scam because
a) it says "Invoice invoice number": I wonder if a script was supposed to substitute a made-up number in place of the text "invoice number"? ;-)
b) although it is signed with a British-sounding person's name, with job title "Finance Assistant", and there is an apparently genuine postal address and phone number, one rather important thing is missing: the company name.
c) the tautology: "a late payment fee will apply if payment becomes overdue by 14 days after which a late fee applies"
I had one a few weeks ago which had a genuine company name, as well as postal address, phone number, web site link and (apparently) sent from an address in their email address. I'd have thought it plausible except for the fact that I have never dealt with that company.