Emails are viewed in a variety of formats, but as a general rule of thumb, keep the #subject line to 50 characters or less. A related issue to be aware of is that many mobile email tools cut off subject lines at 25 characters — and 66 percent of emails are now opened on mobile devices. http://www.entrepreneur.com/article/246160
Once you’ve gotten customers to open your emails, it’s important to keep their attention. Bad content is one of the top reasons people unsubscribe from 25 percent of emails. Content must be engaging and worth your readers’ time. Just because you can get them in the door doesn’t mean that they will stay. Good content leads to brand loyalty and increased referral business.