I'm using one of the models here . It's working for voice instructions for navigation in Organic Maps. It's also showing in the text to speech menu in the android settings.
The app it's based on (SherpaTTS) already exists on fdroid repos, but the APKs on that link has the model embedded already.
Other people already mentioned Roundcube and Snappymail, which are good options already.
There's also Cypht if you want a different approach. It combines multiple accounts into same interface so you can have a unified inbox.