de-mail offers end-to-end enctyption with gpg and Mailvelope as add-on. Generally it works. However, if a key is generated on the GnuPG Smart Card V3.3, it will work. The public key can be uplodaed in Mailvelope. So Messges in de-mail can be encrypted, as it should be.
The problem is comming up, if the private key is only on the smart card. This should be, fore security reasons the case. However, Mailvelope does not recognize the smart card. Therefore, decryption fails.
If not smart card is used, and private and public key are stored in Mailvelope, gpg can be used inside de-mail.