Roger Thomas writes:

And that's a big, honking clue that maildrop is not integrated properly into your system.

Neither Courier-IMAP, nor maildrop, would ordinarily have any knowledge of what you keep in LDAP, unless explicitly configured accordingly.

Furthermore, that version of Courier-IMAP is over two years old, and is prehistoric, in Internet time. The very next version had a complete overhaul of the authentication library; the common authentication library in Courier-IMAP, and maildrop, was factored out into a separate package, courier-authlib.

The best advice for you is to download and install courier-authlib, and configure its LDAP module until it works with your mail account schema, then upgrade to the current version of Courier-IMAP and maildrop. As long as you have properly set up courier-authlib and it works with your LDAP schema, the current version of Courier-IMAP and maildrop should not have any issues reading account data from LDAP.