[RavSinghChandan]

The command-line tool sets each boolean option with bool(os.environ[varname]), so any non-empty value turns it on, including the values meant to turn it off:

CHECK_DELIVERABILITY=0 python -m email_validator me@example.com      # DNS checks still run
ALLOW_DOMAIN_LITERAL=false python -m email_validator me@[127.0.0.1]  # accepted

The only way to turn an option off was an empty value (CHECK_DELIVERABILITY=). 0, false, no and off (in any case) are now read as false too; an empty value is still false and every other value still true.

Added test_main_boolean_environment_variable (with ALLOW_DOMAIN_LITERAL set to true and false spellings); the false spellings fail before the fix. Changelog entry under In Development