If you use lftp alot, occasionally you will encounter sites that just use ssl to secure transmission, but don’t actually purchase a proper SSL cert for the domain.
You would get the following error message:
Fatal error: Certificate verification: Not trusted
If you are sure its really the intended site (No Man in the Middle Attacks!), you can temporaryly disable certificate verification by the following command at the lftp prompt:
lftp > set ssl:verify-certificate no
To permanently set this for lftp, you could add this to your /etc/lftp.conf or in your home directory ~/.lfptrc file.