Just in case anyone else finds their way here, I have managed to get this ~hopefully~ working on a site.
In the source for this plugin (Plugins -> Editor -> Select plugin to edit -> Checkmail Validation for Contact Form 7) I have changed the function wpcf7_checkmail_validation_filter to the code below:
function wpcf7_checkmail_validation_filter( $result, $tag ) {
$type = $tag['type'];
$name = $tag['name'];
$values = $tag['values'];
$_POST[$name] = trim( strtr( (string) $_POST[$name], "\n", " " ) );
if ( 'checkmail' == $type ) {
if ( '' == $_POST['checkmail_'.$name] ) {
$result->invalidate($tag, wpcf7_get_message("invalid_required"));
} elseif ( $_POST['checkmail_'.$name] != $_POST[$values[0]] ) {
$result->invalidate($tag, wpcf7_get_message("invalid_email"));
}
}
return $result;
}
It seems that the WPCF7 API changed or something, I don’t know. But after making this change the plugin works on my WP 4.1 install.
Edit: Here’s a link to a gist that’s a bit easier to read: https://gist.github.com/Riccobab/31841497978a89b873f1#file-wpcf7_checkmail_validation_filter-php