Public-key of the received signed mail can be stored as follows:

// load a mail or retrieve it using POP3 or IMAP
MailMessage receivedSignedMail = new MailMessage();

// store signer certificate
if (receivedSignedMail.Signers.Count > 0)
{
    // get the first signer certificate
    Certificate signerCert = receivedSignedMail.Signers[0].Certificate;

    // save the certificate to a file
    File.WriteAllBytes("path.cer", signerCert.GetRawCertData());
}

The saved Public-key can be load and used for encryption as follows:

// load certificate
Certificate cert = Certificate.LoadDer("path.cer");

// prepare a mail for encryption
MailMessage encryptedMailToSend = new MailMessage();
// ...

// encrypt the mail
encryptedMailToSend.Encrypt(cert);

For information about certificates, signing and encrypting please visit our blog or Wikipedia.