Send to yourself and check in Outlook/ Gmail/ Mobile view
Untick "Can send multiple times to one person" (keep "view in browser" ticked for Newsletters)
Check whether images in both editors are the correct size (for legacy emails only)
Add links to the images (most leads will be checking your emails on mobile, therefore worth considering for small and medium sized images)
Add description to images (otherwise your email might end up in a spam folder)
Check whether ALL links work (including image links)
Check whether all emails have been delivered: Analytics >> Email Jobs
To check Report go to Analytics>> Email jobs>> View email report