MailDroid allows you to sign and/or encrypt your emails prior to sending using either S/MIME, PGP/MIME, or PGP Inline mode using the Flipdog Solutions Encryption Plugin available free on the Google Play Store.

Algorithm options in MailDroid Signing with S/MIME Signing with PGP/MIME or PGP Inline Encryption with S/MIME Encryption with PGP/MIME and PGP Inline

You can also sign and encrypt an email by clicking both checkboxes.

To summarize, MailDroid will allow you to choose one of two algorithms to sign and encrypt your mail. This is an advanced feature and users should read up on the basics of each approach prior to using them.

In the future, more options may be added to this feature.