[Snowhog]

You could write two additional plugins (give each a unique plugin name) using the code with only a slight modification.

Plugin to deny registration if email address contains any colons preceding the @

$this->validfields['email'][VF_CODE] = '
    $max_colons = 0;
    if ($retval = $dm->verify_useremail($data))
    {
        $parts = explode("@", $data);
        if (is_array($parts) && substr_count($parts[0], ":") > $max_colons)
        {
            $dm->error("bademail");
            $retval = false;
        }
    }
    return $retval;
';

Plugin to deny registration if email address contains any semicolons preceding the @

$this->validfields['email'][VF_CODE] = '
    $max_semicolons = 0;
    if ($retval = $dm->verify_useremail($data))
    {
        $parts = explode("@", $data);
        if (is_array($parts) && substr_count($parts[0], ";") > $max_semicolons)
        {
            $dm->error("bademail");
            $retval = false;
        }
    }
    return $retval;
';

I'm sure that these could be incorporated into the original plugin, but I'm not a coder, so maybe kh99 can suggest how to do that.