I was browsing through this mail server setup the other day and remembered seeing this. Maybe it will help! I use dovecot and postfix myself but maybe it will work out for you with cyrus-sasl2 and sendmail. http://www.brennan.id.au/12-Sendmail...r.html#dovecot
Clip from site..
User Email Accounts
When a standard user account is created on the server, that user is automatically granted access to login to the system and is also able to send and receive emails. The user now really only needs to configure their MUA with the details of the servers address and their account details, and email should be fully accessible.
Creating a standard user account with a false shell stops that account from being able to log into the system (a Linux login), but still allows them to use the system for the purpose of sending and receiving emails. Creating a user account using the following example, is the easiest method for creating email only accounts for friends and family where they do not require login access to the server.
[bash]# useradd -c "Alice Jones" -s /sbin/nologin alice
Email only accounts forbid a user from logging in and accessing their home directories. These accounts may not be suitable if users expect access to their "public_html" directories if they are present.
You may also consider placing all of the email only accounts into an appropriately named group for easy management.