No matter how effective the subject line you’ll always have subscribers who don’t open it for a variety of reasons. Send your email again specifically targeting a list segment of those who didn’t open the first time around. Not only is this a second chance in case they just missed the first email, it’s another opportunity to further split test subject lines as well as send times.
The first autoresponders were created within mail transfer agents that found they could not deliver an e-mail to a given address. These create bounce messages such as "your e-mail could not be delivered because..." type responses. Today's autoresponders need to be careful to not generate e-mail backscatter, which can result in the autoresponses being considered E-mail spam.
Over time, the popularity of email marketing campaigns resulted in personal email inboxes that were overflowing with the latest and greatest offers. Email campaigns grew and people grew tired of them. Laws that allowed customers to unsubscribe breathed new life into email marketing campaigns as customers could easily pick and choose the companies they were interested in.