I think I've got it working with negative look-behind, but try to think of other test cases.

    while( <DATA> ) {
        my( $expect, $string ) = split;
        my  $result = $string =~ 
                m/ @ (?: .*  (?<! [@.] bar) \. )? example\.com $ /xi
            ? 'match' : 'fail';
        printf "  Got result '%s' when expecting '%s' on '%s'\n",
                            $result, $expect, $string
            unless $result eq $expect;
    }

__END__
match  smith@example.com
match  jones@example.com   
match  jones@foo.example.com
fail   jones@bar.example.com
fail   jones@foo.bar.example.com
match  jones@baz.example.com
match  jones@bbar.example.com
fail   jones@bar.example.com.com
fail   rednose@reindeer.org
fail   smythe@exzample.com
fail   teacher@make-an-example.com
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