What do you mean, you didn't get my email?
Back in the day, before there were options for email testing like Email on Acid or Litmus, if you wanted to test your email in multiple clients, you had to do it the hard way. You would send it to various team members and ask them to open it on their assorted smart phones or computers. This was necessary to test emails across all operating systems and email clients. What renders propertly in Windows might not render on a Mac, and what looks good in Outlook might look awful in Gmail, and then there's smart phones... you get the idea.
And nothing was so frustrating as waiting for a screenshot or feedback from someone, only to find out they never got the email. Or maybe they had, but it went straight to their spam folder.
Here's how you can prevent your emails from ending up in the junk folder, or never getting delivered at all: