There are lots of ways to buy an email list, but none of them will actually benefit your campaign. Why? Since the owners of these email addresses didn't explicitly agree to receive content from you, there's no telling how interested they are -- or if they're even a fit for what you have to offer. A bought email list is also in violation of GDPR (we'll talk more about this in just a minute).
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.