A good way to look at it is imagine it’s a puzzle and all of the pieces need to be there and be put together for it to work.
Also, each campaign needs to help out other campaigns. For example, post your email campaign on your social network site, and give the option to join your mailing list. And then on your email you should have your social sharing buttons and urge your mailing list to like your Facebook page, or follow your twitter account, which will increase your reach.
And then of course any other sort out digital, magazine or outdoor advertisement should promote your website which urges visitors to sign up for your newsletter.
Email marketing is now what holds everything together though, it is the communication tool that is most commonly used and by far the most developed in the sense that it can be tracked. No other form of advertisement offers such a detailed report in what has happened after you have launched the campaign.
You can test what works before you launch the full campaign (see subject line testing).Email marketing will even give you an exact return on investment. It is the most cost effective form of advertisement which is why it is the tool holds your whole strategy together.