If you need to add encryption or digital signing to the Thunderbird email app (or other supporting apps) on Android, there's one clear and easy route to success.

How to Add ActiveSync on an Android. Exchange ActiveSync enables mobile devices to synchronize email, contacts, calendars and other items associated with a Microsoft Exchange account.

After adding support for IMAP, POP, and Exchange, plus a ton of features to manage and control your email, we recommend Gmail to manage all your email accounts—even non-Google-y ones—on Android.