dbWebmail depends on PEAR.php, which is fine, but...
You include both mimeDecode and mimePart from the PEAR
mime package, but you don't include PEAR!
Either include PEAR.php in your project, or remove
PEAR mime code and mention it in the INSTALL doc.
I found a PEAR.php file lying around somewhere, put it
in the project root and it worked!