Best IMAP mail server

[Stephen Smith]

We are planning to replace our current mail server which is sendmail 
configured for SMTP only.  We would like to move to an IMAP server with 
more security.  The RH distribution has dovecot as a secure IMAP server, 
but I have heard little about it. 

What would be your recommendations for quickly moving our existing 
server and accounts to a new platform. 

Which IMAP server would you use? 

Which is the easiest to manage? 

What benefits are there for your particular recommendation that set it 
appart from the others?

All input will be appreciated.

Steve.