Gajim does not seem to properly handle invalid/broken/expired 
certificates. The _ssl_verify_callback function in tls_nb.py is called 
by OpenSSL for every certificate in the certificate chain (CA first, 
server certificate last) but always return True whether an error was 
encountered or not.

This forces OpenSSL to verify each certificate until none is left, at 
which points it will call _ssl_verify_callback one last time with an 
error number of 0.

(This behavior is documented here:  man 3 SSL_CTX_set_verify
"If verify_callback returns 1, the verification process is continued. 
If verify_callback always returns 1, the TLS/SSL handshake will not be 
terminated with respect to verification failures and the connection will 
be established."
And can be observed in function 
crypto/x509/x509_vfy.c:internal_verify() in OpenSSL source code.)

_ssh_verify_callback only stores the last error code, which always is 0 
unless an error was encountered in the deepest level of the chain (the 
CA), so gajim will not warn as long as the CA is recognized.


(...)

This problem goes beyond expired certificates. It is also possible to 
edit any existing and valid server certificate by changing the CN 
manually. The certificate's signature will be become invalid and OpenSSL 
will detect it and return errnum 7 ("Certificate signature failure") but 
gajim will not warn and will proceed with the connection anyway...


References:
https://trac.gajim.org/ticket/7252