I personally use python plus AWS SES to handle outgoing mail. Sure you have to have an AWS account, setup the SES feature, and get out of the AWS sandbox but once that's front loaded you have a great little e-mail service.
Any instance on 0.19 or higher will likely continue to have federation issues. The devs reopened this issue: https://github.com/LemmyNet/lemmy/issues/4288
Just think about how far that cat has traveled in time and space to reach my screen what a fuzzy lil guy :)
That's all it takes to be honest.