I have subscribed to Groupon mailers.  They are using one of the top tier Email Service Providers in USA, and got Sender Score certification from Return Path.  According to Return Path, the IPs are having an excellent sender score and Acceptance.

Even though Groupon use double opt-in policies, why is gmail putting some of their mailers in spam folder?  A little more investigation provides some suspected issues within their mailing policies.

There are a large number of spam complaints in gmail for Groupon mailers. This makes Gmail to know that Groupon is either not sending relevant mailers to their subscribers or their unsubscriptions are not being honored promptly. Groupon is also not collecting the frequency of mailers that their subscribers want to receive or their subscribers don’t care to fill it up. This will result in list fatigue and their subscribers instead of unsubscribing, they are finding easier to complain spam!

A large of unknown users also indicate that Groupon is sending mail to subscribers without knowing their email behavior, whether the subscriber is active or inactive. This also explains the reason why Groupon is hitting spam traps with ISPs.

ISPs are using better and smarter algorithms now a days to classify bulk emails based on their own proprietary techniques.  They are relying lesser and lesser on third party certifications like Return Path and others.

Hence ESPs and bulk email marketers have to better adhere to ISPs guidelines and ever changing policies. ESPs are to be better knowledgeable about the ISPs regulations and provide their clients with best practices and guidelines from time to time to better their ROI.