[Dovecot] index file vs. memory

Hello,

we have a large base of mailbox users and while pop3-connections are usually closed after the download of all mails, I noticed hundrets of imap-connections kept idle by the users. This is surely according to the idea behind imap, but I'm a bit afraid of overwhelming memory consumption if there are too many imap-connections opened at the same time. Is the index file kept in memory as long as an imap-connection exists, or is it closed as soon as a certain read or write command has finished?

Regards Marten