SecExMail uses industry standard RSA keys. RSA is the worlds foremost public key encryption system. The security of your personal SecExMail key will vary depending on the key size. Please consult the documentation on the SecEx Key Generator and key size for details.
The session keys size is 256, 64 and 64 for the Twofish block cipher, the ISAAC stream cipher and the One-Time Pad respectively. See the SecExMail composite cipher for details.