It was back in 1996 when Hotmail launched their simple e-mail signature add-on to new subscribers. Simply put, the call to action was "Get your free e-mail at...." - within the first month the new service had garnered 20,000 new subscribers - a few months later it hit 1 million.
Hotmail’s competitor spent $20 million to attract less than half of Hotmail’s subscribers, it spent only $500,000 over a 2 year period to gain 12 million subscribers.
Many of us receive 100s of e-mails in a given day- and yet as I still look through the e-mails I see very few e-mail signatures that have any call to action. This is free advertising and a way to humblebrag quietly. If something catches my eye, I'll be the first to admit - that I will click on it.
Here a are just a few examples of things you can add into your signature on a monthly or quarterly basis (the options are limitless):
New keynote or speech
Either way, it's quick- simple- and something you can do in less than 5 minutes.