It is important for an interviewer to know about your success and failures as it will help them know your attitude towards it. Unsuccessful campaigns or increased number of unsubscribes is common for an email campaign, but your approach to deal with the situation decides it all. This will give them a glimpse of how you will handle the situation, if it happens in the future too. The way you handle an unsuccessful campaign will tell the interviewers about whether you are a perfect fit for the role or not. However, keep in mind that the success of a campaign not only depends on the increase in the number of subscribers, but also on the mindset that it could change.