e-mail apps on Android and iOS

• https://email.faircode.eu/ – FairEmail – I use it on Android
• https://k9mail.app/ – K-9 Mail – an alternative
• https://blog.thunderbird.net/2022/06/faq-thunderbird-mobile-and-k-9-mail/

What to use on iOS?

• https://connect.mozilla.org/t5/ideas/thunderbird-for-iphone-and-ipad/idi-p/21628
• https://www.techradar.com/news/the-best-alternative-email-apps-for-ios-15-in-2022