Email from the advertiser is sent to recipients with a tracking pixel that tags the recipient's browser, in the same way they would be tagged if visiting a webpage.
Any recipient who uses a dedicated mail client will not be tagged.
[1] Email retargeting relies on sending personalized e-mail to an anonymous website visitor.
This is possible thanks to using a DMP (Data Management Platform) or a noCRM email retargeting provider, where cookies collected on a website include the IP address of the visitor (the IP address can be read from the visitors web browser's header information).
This form of remarketing helps increasing conversion by reaching interested audience only.