Another lesson in email marketing that's not discussed in the article is around email frequency. One key to powerful emails is getting the timing right. Sending weekly emails is for some companies too much... and for others too little.

It's imperative to track your email campaigns:

views

clicks

activities / goals

session length

then return sessions

It's also important to give users the ability to set their own timing. Some users want emails with every update (ie on Facebook). Others want daily summaries.