Interesting

Brilliant:

Their emails have a clear call to action and a single link. They’ve refined their communication through years of experience.

Wait a sec, you should put as many URLs in your emails as possible?!

Let’s assume that most users are sending vaguely relevant things to their audience, such that each link has a non-zero probability of being clicked. In that case, if we assume that lots of URLs don’t scare people, then the clicks should go up as the URLs go up.

Oh, no, wait, you should put only as many URLs as are relevant.

Whew, that's better.

The conclusions are common sense:

1. Provide your readers relevant content.

2. Add more relevant content if you’ve got it.

3. Accompany links with concise statements.

4. Add images. Not too many, just enough for flavor.