The following is a list of some email services administration interfaces. These are web-based.
Virtual Exim or vexim
http://silverwraith.com/vexim/
web interface for managing your email domains;
create and remove user account;
uses SQL backend;
delegate domain administration to respective owners;
update their own passwords and mail preferences;
integrated with SpamAssassin
VMC linked from http://www.ex-parrot.com/~chris/vmail-sql/
How to setup virtual users on Open WebMail using Postfix & vm-pop3d
http://www.bluelavalamp.net/owmvirtual/
mentions openwebmail-vdomain.pl module.
OpenWebMail with openwebmail-vdomain.pl module
http://openwebmail.nobandwidth.net/
VishwaKarma
http://kandalaya.org/vishwakarma.shtml
Web-hosting control panel
ISPConfig
http://www.ispconfig.org/
Hosting control panel for HTTPd, FTP, BIND, POP3, sendmail or postfix,
IP addresses and more.
VHCS - Virtual Hosting Control System
http://vhcs.net/
Control panel for shared, reseller, virtual and dedicated server management.
For DNS, Apache, autoresponders, IP addresses, traffic accounting, and more.
Tequila
qmailadmin and vqadmin
http://sourceforge.net/projects/qmailadmin
http://www.inter7.com/index.php?page=qmailadmin
http://www.inter7.com/index.php?page=vqadmin
vqadmin is for adding and deleting domains, modifying quotas, etc.
qmailadmin is for users to manage their own domains.
maybe use SquirrelMail plugin API