Most of the time it's fine and may even increase opens which will ultimately increase conversions.
Here are the subject lines from three emails I received earlier today:
"Elmer -> do you use wireless networks?"
"Elmer, I promise you at least one profitable PPC campaign"
"Elmer, Make $20k in 20 days.. or your money back!"
Two are informative and imply a no-risk profit potential. The first one arouses curiosity. Nothing wrong with any of them. They served their purpose--I opened the emails.
There was a fourth email with this subject line:
"Elmer the Rat and the Popstar"
This subject line differs here from the original only with respect to "bolding" it and placing it in quotes.
I don't know the fellow that sent this email. I receive something from him every couple days or so. He's a very persistent affiliate marketer. Of the approximately 150 emails I've received from him this year this is the only one with such a unique subject line. This is for an affiliate promotion so I don't know if this is his original work or was provided by the group that developed the product. Either way he should have taken a few more minutes reviewing it before hitting the send button.
Fortunately for the gentleman I'm fairly thick skinned and reasonably understanding of mistakes. So I won't be unsubscribing because he called me a "Rat" or for that matter a "Popstar"--which I'm not.
Suppose, however, I was a wide-eyed newbie with a tender psyche...
Your email subject line--including its punctuation and formatting--is your continuing point of contact with your potential customer.
It can make you or break you.
Just something to think about...