tls-untrusted

tls-untrusted is a variable defined in `tls.el'.
Its value is

"- Peer's certificate is NOT trusted\\|Verify return code: \\([^0] \\|.[^ ]\\)"

Documentation:
Regular expression indicating failure of TLS certificate verification.
The default is what GnuTLS's "gnutls-cli" or OpenSSL's
"openssl s_client" return in the event of unsuccessful
verification.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.