What kind of reaction do you want when you send an email or other message to your customers?
The reactions are likely to fall into 1 of 3 categories in the customer's mind
Delete - Why?...because based on the last few messages you've sent me I have no time now nor later to open it. ..even to un-subscribe
Later - Why?..because you generally send me good content that I like, but I haven't got time right now to give you my full attention while perhaps I'm mobile
Now - Why..because it's Timely, has the right content, recognises the device I'm using and as a result is Relevant and hence is of Value to me