let pass emails encrypted to a not known key

If an email is encrypted to a private key that bitmask don't know about it should let it pass as it is. Right now is failing:

[2016-04-01 13:20:57] ERROR    - L#729  : leap.keymanager.openpgp:decrypt - Failed to decrypt: Failed to encrypt/decrypt: [GNUPG:] ENC_TO xxxxxx 1 0
[GNUPG:] ENC_TO xxxxxxx 1 0
gpg: encrypted with 2048-bit RSA key, ID xxxxxx, created 2014-06-18
      ""
[GNUPG:] NO_SECKEY xxxxxxx
gpg: encrypted with RSA key, ID xxxxx
[GNUPG:] NO_SECKEY xxxxxxx
[GNUPG:] BEGIN_DECRYPTION
[GNUPG:] DECRYPTION_FAILED
gpg: decryption failed: secret key not available
[GNUPG:] END_DECRYPTION
.
[2016-04-01 13:20:57] WARNING  - L#628  : leap.mail.incoming.service:_decryption_error - Failed to decrypt encrypted message (Failed to encrypt/decrypt: [GNUPG:] ENC_TO xxxxxxxx 1 0
[GNUPG:] ENC_TO xxxxxxxxx 1 0
gpg: encrypted with 2048-bit RSA key, ID xxxxx, created 2014-06-18
      ""
[GNUPG:] NO_SECKEY xxxxx
gpg: encrypted with RSA key, ID xxxxx
[GNUPG:] NO_SECKEY xxxxxxxx
[GNUPG:] BEGIN_DECRYPTION
[GNUPG:] DECRYPTION_FAILED
gpg: decryption failed: secret key not available
[GNUPG:] END_DECRYPTION
). Storing message without modifications.

(from redmine: created on 2016-04-08)