Hi,
When you configured multidrop, have you configured the retrieved mail to be internally forwarded to an user of your system ?
Because if not, the mail is probably in the root mailbox or in an unreachable mailbox. In this case, create an user 'toto' (for example), and make a forward to user toto in multidrop panel.
In any case, it is more secured to access toto mailbox than root mailbox with a mail client, because of the password being send in clear text...
Hope it helps,
Erika