The humble subject line has long been an overlooked element of email marketing, often taking a back seat to design, copywriting, imagery and call-to-action refinement during the development of a campaign.

Yet, despite its lowly status on this list of specialist areas, the subject line has actually always been an important factor in email marketing. A good subject line, basically, can make the difference between an email flyer being opened and acted upon or ignored altogether. You can have the best looking email marketing in the world but without a decent subject line to back it up it’s never likely to fulfil its potential.

Now, with mobile devices becoming ever-popular, the subject line is more important than ever to email marketing because of one simple fact;

Smartphone inboxes only display subject lines and from fields.

No images. No preview panes. It’s all down to your subject line. So it better be good.

With the explosion of smartphone use in recent years - the ubiquitous iPhone and Blackberry, along with the rapidly phenomenal Android OS phones (which grew 350% in the last quarter here in the UK) - more and more of us are accessing our emails on the move, or even opting to use smartphones as in-office business devices due to their ever-increasing performance benefits. Smartphones aren’t the preserve of business or the technically-minded, they’re for everyone. And it seems everyone has got one.

Microsoft, which has stayed on the fringes of the smartphone market for the last few years, is moving closer to the launch of its “Windows Phone 7″ operating system - a hi-spec platform that will doubtlessly be backed up by a gargantuan marketing budget when it emerges before 2010 is out. The new Windows offering will present users with yet another super-capable handheld option - and it’s only going to get bigger.

Of course, email on the move simply presents yet another way to use email marketing to communicate effectively and efficiently with your target market. So it’s unquestionably a good thing.

But next time you’re crafting a campaign that will spring into life on iPhones, Androids and as yet unimagined super devices, make sure you don’t forget the importance of the element that has been around from day one of email - the subject line.