SSL Handshake Failure during Client Certificate Verification in Dovecot

A Dovecot configuration requires mutual TLS (mTLS) to authenticate clients via certificates. While the ssl = yes and ssl_cert directives are configured for the server identity, the system must also validate the incoming client certificates against a trusted root.

The current uncertainty involves the behavior of the ssl_ca directive when managing a complex trust store. If the CA bundle is incomplete or the backend (OpenSSL vs GnuTLS) handles the chain verification differently, the handshake may fail without a clear indication of which certificate in the chain is missing.

• How does Dovecot handle the validation of client certificates when the ssl_ca directive points to a directory of certificates versus a single concatenated bundle?
• What is the expected behavior when a client provides a partial chain that is not fully mirrored in the server's trust store?