Email providers all render HTML differently. While an email may look great in Gmail, it could be jumbled up in Yahoo Mail or Outlook. Unless your email is 100% text based, it is smart to test your emails on a variety of email servers to ensure the design looks right.
At On-Target! we use the HubSpot platform for emails which gives us a preview screen, so we can see exactly how our emails will look. It also allows us to segment lists, schedule sends, and collect key metrics.
To see how email are done the inbound way, watch the video below.