Mailvelope is not available for Android but there are some alternatives with similar functionality. The best Android alternative is Tutanota, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Mailvelope and five of them are available for Android so hopefully you can find a suitable replacement. Other interesting Android alternatives to Mailvelope are CTemplar (Freemium, Open Source), FlowCrypt (Freemium), Psono Password Manager (Free, Open Source) and Lockbin (Freemium).
CTemplar is a security and privacy focused email provider based in Iceland. They use E2EE (end to end encrypted encryption) to keep your conversations private and secure. Their service offers onion address for those who want the best possible privacy and security.
FlowCrypt (formerly CryptUp) lets you Encrypt Gmail with PGP in 60 seconds. Easiest end-to-end encryption for Google Email. FlowCrypt supports attachments, drafts, conversations.. just like Gmail you are used to.