This really depends on the culture of the mailing list. Discourse has fairly robust email support (for notifications, and if configured, for replies and email-in to start new topics), but it is still fundamentally web-centric in the way that it views the world. There will be clashes for people who are 100% email-centric.
Do you have support from the “powers that be” at said mailing lists to make such a change? Are they asking for such a change? We are very open to working with a partner on migrating mailing lists and further enhancing the mailing list support in Discourse, but it very much requires solid support from the leadership and a significant part of the community.
There’s a lot of friction involved in changes for groups!