In my opinion email marketing is probably the hardest thing to get right...
There are two schools of thought - those who churn and burn ... and others who do their best with building a relationship... I'm of the latter school of thought - but each to their own...
My best advice is to provide value above all else as the key is to build trust.... with trust comes folks prepared to invest in your recommendations as an affiliate or to try your product if you are a vendor...
One tactic that I find that works well is to keep your messages short but to refer to hidden pages on your site which provides a great, helpful article - or using a free membership site set-up... It sound counter intuitive but its not as it allows you to build a relationship, with your audience realizing that wow - this is not openly available...
The key however is to identify your target audience - the avatar... understand the pain points on a deep level, live it, breathe it and work to find what eases that on a personal level.... Its not easy to step into other peoples shoes but if you can regardless of gender, age etc - you could be amazed at what you achieve... but like I alluded to empathy is the key here .... and above all building your list is hugely important in this business...
BTW - I use a self hosted autoresponder solution using Amazon SES (or similar wrt smtp such as mandrill etc) which I recommend it if you can do it as it allows you to scale up big time with ~10K messages per day for free (way better than the big names) and you're not held hostage by third party solutions in many ways... If you do that remember to use DKIM, SPF and you'll get delivery results better or equal to the big email providers...
Regarding metrics I find email marketing is by far the biggest means to ROI with conversions as high as 40% (from a recent campaign with a not so large list, highly targeted suffice to say it brought in the goodies) versus cold traffic at around 2%-5% if you're lucky....
Good luck!
