When I try to register someone I get the following error messages

Warning: require_once(Zend/Mail/Transport/Abstract.php) [function.require-once]: failed to open stream: No such file or directory in /home/xxxx/public_html/cms/concrete/libraries/3rdparty/Zend/Mail.php on line 26

Fatal error: require_once() [function.require]: Failed opening required 'Zend/Mail/Transport/Abstract.php' (include_path='.:/usr/lib/php:/usr/local/lib/php:/home/next4/php') in /home/xxxxx/public_html/cms/concrete/libraries/3rdparty/Zend/Mail.php on line 26


the file exist on the server.
The User is also in the database, but no emails sent out.

Everything else works so far as expected

Website:http://nextdimension4.us/