Yes, ask them to put in a subject
joking apart when you try to send an email without a subject in OE it pops-up a message, no subject, etc. So if they are also using OE they should at least know about it.
This is the work of the Heuristic function in the Internet Mail provider, so you can customize the heuristics to either Lower the sensitivity or customise the checks, see images.
Personally I hate emails without subjects as it is a common spam/scammers ploy to get you to open them with potentially serious consequences. The problem you can't easily tell if this is from a friend who has left the subject blank or a scammer with a forged from address without opening it or looking at the properties, message source.