Installation
Download the latest tarball of the Dovecot source:

Configure Dovecot with support for vpopmail, compile and install it. If you want POP3 as well as IMAP, you need to specify it in the configure options.

Create a user and group for Dovecot to run as:

Configuration
Dovecot has installed a /usr/local/etc/dovecot-example.conf file, which you'll have to rename to /usr/local/etc/dovecot.conf:

Authentication
To enable authentication with vpopmail, edit /usr/local/etc/dovecot.conf and uncomment:

You'll also want to make sure that pam is disabled in /usr/local/etc/dovecot.conf:

Since the vpopmail user is uid 89, we need to adjust a few more settings in /usr/local/etc/dovecot.conf:

Courier by default uses "INBOX." as the IMAP namespace for private mailboxes. Configuring Dovecot the same way can make things easier when configuring clients and also when switching existing Maildirs from Courier to Dovecot. There are two ways to accomplish this:

• Using the same INBOX. namespace as Courier. With this method, Mail.app (at least in OS X 10.3) required that you specify INBOX as the IMAP Path Prefix when configuring the account.
  
  namespace private {
  separator = .
  prefix = INBOX.
  inbox = yes
}
• Create the INBOX. as a compatibility name, so old clients can continue using it while new clients will use the empty prefix namespace.
  
  namespace private {
    separator = .
    prefix = INBOX.
    inbox = yes
  }

  namespace private {
  separator = .
  prefix = INBOX.
  inbox = no
  hidden = yes
  list = no   # for v1.1+
}

Self-Signed SSL Certificate
Dovecot by default does not allow plain-text logins so you'll need to have an SSL certificate for it use. You can use Dovecot's instructions or you can create a self-signed certificate using the Apache SSL instructions. If you do it the Apache way, copy the certificates to a location where we'll configure Dovecot to read it from:

Then modify /usr/local/etc/dovecot.conf to find the files:

However, if you did need to use plaintext auth with Dovecot, you can modify /usr/local/etc/dovecot.conf to allow it. Please, don't let users connect over the Internet like this though!

Daemontools Startup
The DJB Way website has a page on running Dovecot through daemontools. These instructions are adapted from that.
Firstly, create a directory to hold the service:

Then create the /var/service/dovecot-imapd/run file with the following contents:

Make the run script executable and then set up the logging service:

Edit /usr/local/etc/dovecot.conf so that it sends log messages to /dev/stderr where multilog can catch them.

Make the symbolic link in /service to start Dovecot:

After about 10 seconds, confirm that it is running:

Courier to Dovecot migration
In general, there's not really much you need to do to migrate existing Maildirs from Couier-IMAP to Dovecot. However, one thing I did notice was that folder subscriptions do not copy from Courier's Maildir/courierimapsubscribed file to Dovecot's Maildir/subscriptions file. Depending on your mail client, this might not be an issue. Mail.app, for instance, picked up the new folders even though they weren't listed in Maildir/subscriptions. On the other hand, SquirrelMail didn't see them unless you manually subscribed to them.

The Dovecot wiki has a wiki Courier migration page has some migration details that have an impact for POP3 users and has a Courier-to-Dovecot migration script to take of some POP3 UIDL issues. The script also can convert Courier's Maildir/courierimapsubscribed file to Dovecot's Maildir/subscriptions format (basically removing "INBOX." from the entries).