Discourse offers a "mailing list mode" user preference. When enabled, every single post published in a Discourse instance will be sent via email to the user (respecting users' permissions).

What Discourse lacks is the possibility to enable mailing list mode per category, which could be a way to support the transition of existing mailing lists to Discourse.

Asking at Discourse upstream, implementing this feature doesn't seem to be a big amount of work, although it is not a priority for the Discourse maintainers. If our testing of mailing list emulation shows that the results are not good enough, we could explore the commissioning of this feature.