According to the UK DMA, marketers report improvements in deliverability after GDPR went into effect.
I mean, thank you UK DMA for doing the research. Of course deliverability is going to improve when you send mail to those people who have expressly opted into your mail.
76% report an increase in open rates in the past 12 months; 75% say click- throughs are higher; 51% say ROI has risen. Opt-out rates have decreased according to 41%, and 55% say spam complaints are also down — further evidence of the GDPR effect. DMA UK Marketer Email Tracker 2019
We often gloss over the most important best practice: send mail to people who ask for it. Many of the data hygiene recommendations are all about making sure that the recipients still want your mail.
Recipients, and ISPs, expect senders to have permission before they start blasting out emails. Most best practices are a way to confirm permission. The others, things like authentication and the like, they’re ways for ISPs to identify your mail streams so they can assign reputation and effectively deliver mail.