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.
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