X509_PURPOSE_SMIME_SIGN
X509_STORE_CTX_set_purpose(ctx, X509_PURPOSE_SMIME_SIGN);
{ X509_PURPOSE_SMIME_SIGN, X509_TRUST_EMAIL, 0, check_purpose_smime_sign,
X509_PURPOSE_SMIME_SIGN, /* purpose */
X509_PURPOSE_SMIME_SIGN, /* purpose */