Groupon, which sent about 388 emails per user last year, was the most popular mailing list that consumers wanted to unsubscribe from, according to a new report from Unroll.Me, an app that lets consumers mass unsubscribe from email marketing lists. LivingSocial followed behind Groupon. Facebook was No. 3 on the list, Meetup No. 4 and Twitter was No. 5 on the list.