What you put into your email, when you send it, and who opens it can make or break your email campaign.

Here's a closer look the importance of each of those factors, according to the following Constant Contact infographic.

What are you putting into your email? About 20 lines of text and three or fewer images result in optimal email campaign click-through rates, reports Constant Contact.

When to send your emails depends on the industry you're in. Mondays at 7AM are ideal for restaurants. For accountants and financial advisers, however, Tuesdays at 6AM works best.