Horde/IMP won’t create sent mail folder. This is what the server said: Invalid mailbox name

If  you face this problem while creating the sent folder in horde on a linux Plesk server this means  that the php has been  upgraded to 5.3.6 or later version on  that server  and some of the horde webmail namespaces are not compatible with  the php  5.3.6 version  and hence you were getting that error every  time you try to  create  a sent folder.
As a work around you need to edit the below two files to fix this error :
/usr/share/psa-horde/imp/lib/IMAP/Client.php line 576
function namespace($additional = array())
function plnamespace($additional = array()


/usr/share/psa-horde/imp/lib/Session.php line 295
_SESSION[‘imp’][‘namespace’] = $imapclient->namespace($user_namespace);
$_SESSION[‘imp’][‘namespace’] = $imapclient->plnamespace($user_namespace);

Restart the httpd service on the server.

/etc/init.d/httpd restart

Now, just refresh your Web-Browser and then try to create a sent folder by following the below steps :

Click on Options and then, within the Mail category, click on Personal Information.
Scroll down to the bottom of the page, and click on the save sent mail ? checkbox and finally click on save options.

That’s all just refresh your web-browser again and you will now be able to access the sent mail folder.