Решено!
//..
socket = new QSslSocket(this);
QSslConfiguration ssConf = QSslConfiguration::defaultConfiguration();
QList<QSslCertificate> ca_list = ssConf.caCertificates();
QList<QSslCertificate> ca_new = QSslCertificate::fromData("CaCertificates");
ca_list += ca_new;
ssConf.setCaCertificates(ca_list);
ssConf.setProtocol(QSsl::AnyProtocol);
QSslConfiguration::setDefaultConfiguration(ssConf);
connect (socket, SIGNAL(sslErrors(const QList<QSslError>&)), this, SLOT(ssErrorHandler(const QList<QSslError>&)));
//..
void BlSmtpSsl::ssErrorHandler(const QList<QSslError> & errList)
{
qDebug() << "----sslErrorHandler: ";
foreach (QSslError err, errList)
qDebug() << "ssl error: " << err << " | " << errList;
this->socket->ignoreSslErrors(errList);
}