Test. Different email clients and mobile devices display emails differently. Send test emails to colleagues, or use a testing program to make sure your emails are going to look good on screens big and small. Testing reveals design mistakes before it's too late, and testing programs can predict whether or not a campaign will get caught in a spam filter. You could even set up accounts with a few different email services for easy testing. Avoid sending one big image as a campaign, and cover your bases with a plain-text option for every email.
When it comes to deciding how to craft that perfect subject line, there appears to be really only one area to avoid: the subject line of 60 to 70 characters. Marketers refer to this as the “dead zone” of subject length. According to research by Adestra, which tracked over 900 million emails for its report, there is no increase in either open rate or clickthroughs at this 60-to-70 character length of subject line.
The Preheader references the area before the main content. It's an excellent opportunity to grab a bit of extra pre-content web real estate. It's got a degree of prominence in the inbox, so it would be a waste not to use it for promotional purposes. Try popping a couple of links in here. Some companies find that they get the most clicks on links within their preheader
Think about mobile. If a campaign doesn't show up on mobile devices, it's not going to perform very well. Everything you send should be mobile-friendly. Check out ReturnPath's "Email in Motion" infographic for some data that might affect the way you design your emails. One of the highlights: According to the study, 63 percent of Americans and 41 percent of Europeans would either close or delete an email that's not optimized for mobile. Might be time to start using a responsive template.
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