Presumably you want the recipient to read the message. That's why you sent it. It would behoove you to select a subject line which conveys to your reader the purpose of your message. Otherwise your reader is likely to ignore it for being too vague and uninteresting.
Here are some actual bad subject lines I've seen.
- Customer question
- Question about Windows XP
- New question
- Help needed
- Any help will be most appreciated
- A tough problem!
- Some questions on Windows
- Urgent: Query regarding Windows
- Request for information - URGENT!!
- URGENT URGENT URGENT
Suppose your Inbox had a hundred messages that all looked like this. Would you bother reading even one of them?
I'm sure the subject line makes perfect sense to the person who sent the message. They have only one customer with a question, so "Question from my customer" captures the issue perfectly. But if you send this message to a list with 500 members, those other 499 people most likely will not know what your message is about based solely on the subject line.
A good subject line would include enough information about the question so that the recipient can decide whether to read further or whether it's something they can't help with. For example, "Question about generics," or even better, "Question about covariant types in generics."
Remember, choose a subject line that is meaningful to the person you're sending it to. It's only polite.