Solus emails are fairly popular amongst local communities, usually local directories and news outlets allow businesses to place full-page advertorials within an email campaign to be sent to their subscriber base as a way to earn extra revenue. I question the GDPR ramifications on this one!
Another option is paid-for subscription-based emails. This option hasn’t really become that popular, and you can see why; how do you stop someone sharing the email content? It’s easy to lock down web content, but emails can be shared or forwarded.
Lastly, a way to monetise emails is through advertising. Think of it like ‘Google Adsense for email’, allowing people to place ads within emails instead of on websites. I’m surprised there isn’t a go-to service for this, or an option within some smaller ESPs to allow this, as it makes sense as an extra revenue generator.