[nefariousoctopi]

> This is a core feature of every ad platform I've seen and is absolutely not a violation of the GPDR.

I agree with you on this part. It is not a violation of GDPR on the ad platform side since you, as the data controller, are responsible to obtain a permission from the end-user. The ad platform is a data processor defined under GDPR. I am sure that the agreement between you and the ad platform is stating that you have a permission to use the email addresses for targeted advertising purposes and bear the full legal responsibility if not.

> since users are giving consent when they signup.

See Nextgrids comment. Yes, the GDPR admittedly lacks on the enforcement side and yes, I agree that this is a common practice, but that does not make it legal. Not for a data subject residing in the EU.