The Check-up section allows you to perform one last check-up before sending the email message.

You can perform the Check-up feature:

from the Overview page, click on the Checkup tab
Using the Message check-up tab in Overview page (click on the issues highlighted to access the check-up page)
from the menu in the window of the message preview

The Summary tab will give you important information on the Message's health such as:

Blocking issues:

Can the message be sent? Are there any other blocking issues?

Potential issue:

Do links have any reputation issue?
Will clicks on links be tracked?
is the unsubscribe link included?
Does the message exceed the maximum size?
are there broken links or missing images?

Improvements:

Add Preheader
is any Web analytics tracking code inserted?

For all the issues in the Summary tab that don't block the success of the email itself, i.e. Potential issues and Improvements, you can decide to always get notified, never or to snooze the alert message for the current email.

To pick which alerts you want to receive, click on "Manage alerts" on the top of the page and then modify the slider.

The Check-up section also allows you to:

check links validation and reputation (link check)
check if your email content may be classified as spam (Spam check)
analyze if the HTML code has any errors (code review).

Be sure to give it a try before sending your message!