Holy cow, I wrote a book!
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
Here are some actual bad subject lines I've seen.
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
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.