If you’re thinking of sending back-to-school emails to entice customers, a new study suggests you may be wasting your time.

While the number of brands blasting holiday-themed email campaigns grew by 13.4% in 2015, the amount of those emails that were actually opened decreased by 5%, according to Yes Lifecycle Marketing. The agency tracked the year-over-year performance of email marketing campaigns for 30 major brands such as Amazon and Target in both Q3 and Q4 of 2014 and 2015. While open rates for Q3 2015 were higher year-over-year, they decreased by 13% moving into Q4, coinciding with a 29% boost in send volume during the same period.

“Holiday-themed email campaigns are a great way for brands to engage consumers in a timely and topical way, but we saw some diminishing returns as more brands were competing to attract customers’ attention with the same tactics over the same period,” Michael Fisher, president of Yes Lifecycle Marketing, said in a press release. “These campaigns can still be highly effective and engaging, but marketers need to think more strategically about how to relate to consumers as individuals, instead of simply latching onto holiday themes.”

The study found that marketers sent more back-to-school emails in 2015 than in 2014, but the average open rate decreased by 3.4%. However, Black Friday-themed emails issued over Thanksgiving weekend generated higher open rates over the same period. Only 10% of retailers sent emails promoting Green Monday, typically the second Monday in December and the third largest online shopping day of the year.

“More is clearly not better when it comes to email,” said Michael Iaccarino, CEO of Infogroup, the parent company of Yes Lifecycle Marketing. “By using intuitive technology and experienced service partners to propel data-driven programs, retailers are able to consider the specific needs of their audience and meet business objectives.”