I think you should consider migrating to dovecot. It is much faster than courier. I too migrated from courier to dovecot.
When the number of emails in your inbox is sizeable in number, with courier it takes ages to open and even fails to open many times, but using dovecot the mails in dovecot is displayed in few seconds with 10000 mails in the inbox.
The main reason is dovecot uses a index for the mails and courier doesn't.