The preheader: The great forgotten of email marketing

We are used to seeing a text before the content of the emails we receive. In this text we usually find the link to the  mirror page and, less frequently, down links and short texts as a synthesis of what we will find developed below. This small summary-headline is what we call the preheader. We encounter it far less often than we would expect after seeing what it is for.
This text is usually placed at the beginning of all content and may sometimes be hidden when the email is opened, as its function is to add to the subject. In many email clients, before opening the email, we not only see the subject and sender, but also some text from inside the message. Let's pay attention to how much space the preheader takes up in the inbox:

 
And what about mobile? Because this is a desktop capture, but mobile devices are starting to be the main way to open emails. Let's see what the inbox looks like on iphone:

That being the case, it is not a good idea to ignore the preheader and miss the opportunity to impact the user with relevant text, rather than what we often find:

"If you can't see this email correctly, click here".

If, for design reasons, a preheader text breaks our initial idea, we can always opt for hide it in the message once it has been opened. We can do it like this:

 
Note: In the colour of the text we have to put the background colour where it is located.
Or so:

The number of characters in the preheader displayed by each email client is different, so we must adjust the copy to a useful solution for all options. We should not give too much responsibility to the preheader, as there are email clients that will not show it, but take advantage of the opportunity in those cases where it can add impact.
In addition, the preheader text can function in different ways... interacting creatively with the subject matter, intriguing about the content of the message, adopting a more informative role in the case of non-descriptive subjects ... We can test different uses of the preheader for information on which may work best.