Thanks for the interest in Thundermail! We're exploring adding a family plan, so please add an upvote and/or a comment to the suggestion for it on our Ideas page! https://ideas.tb.pro/p/family-plan
ThunderbirdFamily plan | ThunderbirdWhat you’d like to seeI would love to see a famuly plan to bundle 5+ accounts to share a custom domainWhy it mattersShare a custom domain with family and save moneyYour use caseMy family 5 persons should be able to share one custom domain and it should be cheaper.