[imp] How to change from Net_SMTP to Horde_Smtp

[Michael Cramer]

after sucessfully install Horde_Smtp and uninstalled Net_SMTP i got an
error in my logs when trying to send a message through imp.

the error is:
[27-Aug-2013 07:53:19 UTC] PHP Fatal error:  Class 'Net_SMTP' not found in
/srv/www/xxx.xxx/pear/share/pear/Horde/Mail/Transport/Smtp.php on line 259

is there a configuration option that must be changed?