Email retargeting

Email retargeting can refer to either:

Retargeting an audience with display ads after they have read an email from the advertiser
Emailing a website visitor after their website visit

Email retargeting is a commonly used form of Digital Marketing.

Display-ad based

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.
Email retargeting is only effective if the email is opened in a web browser, where the tracking pixel will function. Any recipient who uses a dedicated mail client will not be tagged. Additionally, Gmail's decision to cache images means that this method of advertising does not work with gmail addresses.

Email-based

Email retargeting relies on sending personalized e-mail to an anonymous website visitor.

Visitor browses a website, views a product and leaves
Cookies from the website matches the user to an email address
Visitor receives a personalized offer via e-mail message concerning the product they viewed

This is possible thanks to using a DMP or a noCRM email retargeting provider, where cookies collected on a website include the IP address of the visitor. The IP address is then matched against the IP addresses previously collected with form submissions that include an e-mail address.
This form of remarketing helps increasing conversion by reaching interested audience only.