Booster emails are available on the Tempting Giveaway, Leaderboard Giveaway and Startup Prelaunch. They serve as a "reminder" or "booster" for your participants to keep inviting their friends.
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When going on the "Boost campaign" step what you see is this:

There, you may set number of days of delay before emailing a participant again (that number cannot be less than 5) and choose whether you want them to be sent automatically or manually. 

Now here is the tricky part!

What you have to do, is "Edit booster Emails" and edit both segments. Depending on the template, these two segments can be referred to as "More than average, Below average" or "Top of the Waitlist, Rest of the participants" but the concept is the same.

You need to review both segments and edit them so they can appear as "Reviewed" on the menu:

and of course, don't forget to Save your campaign!

So, booster emails do work, provided that you edit both segments, as mentioned! 😊

​*Note: If you set them to "automatically sent", please read when they are triggered here: When are the automatic booster emails sent? Still, if you choose the manual option, instead, then a "Boost" button will appear in your campaign's dashboard after the equivalent number of days you set in the "Booster emails" step of the campaign wizard ("Email them after X days" section).
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That's all! 

Let us know if you need any further help!