Here’s a neat little tool you might want to bookmark. SubjectLine.com’s free Rating Tool will assess your email subject line and grade it out of 100, based on marketing effectiveness and deliverability. The site says that results from this tool are based on over 3 billion email messages sent and tracked via the company’s partners and clients, and that over 800 unique rules are tested.
You seem to get one free shot at it, then you’ll need to let them have your email address and details for further tests, which is fair enough. The subject line for this email scored 82, with suggested improvements being to reduce the character count for mobile devices and to add some ‘subtle urgency’.