Pre-header text is like an extra subject line for your email.

Who wouldn't want that?

It shows on mobile devices just below the subject line. On some desktop clients like Gmail, it can show to the right of the subject line, depending on display settings.

Used carefully, it can mean the difference between an email open and not.

It must be padded with special characters to stop unwanted text appearing beside your pre-header. 

Other platforms have a similar approach.

PS Most mobile devices will show 32 letters or less, so keep the visible text under 32 if you can.

Type pre-header text, then click Pad.

Even top brands trip up on email 101

Only one brand below managed to inbox correctly.