SSL_set_verify(3) explains:

SSL_VERIFY_FAIL_IF_NO_PEER_CERT
  Server mode: if the client did not return a certificate, the TLS/SSL
handshake is immediately terminated with a "handshake failure" alert.
This flag must
  be used together with SSL_VERIFY_PEER.

  Client mode: ignored

SSL_VERIFY_CLIENT_ONCE
  Server mode: only request a client certificate on the initial TLS/SSL
handshake. Do not ask for a client certificate again in case of a
renegotiation.
  This flag must be used together with SSL_VERIFY_PEER.

  Client mode: ignored

The SMTP connection here uses a OpenSSL socket in client mode,
suggesting invalid/ignored flags is rather misleading.