newbie question re: needed mail functionality

[faq100_(_at_)_earthlink_(_dot_)_net]

I've been using OpenBSD for several years now as gateway/firewall/webserver but never as a 
mail server.  I have several functionality requirements and am unsure if these can be met with 
the default sendmail/popa3d combo included w/ 3.1.  I was hoping somebody can advise me 
on whether I need some additional software and if so what the recommended candidates from 
ports may be.  I'm pretty sure this all pretty basic. 

Functionality needed:
- I have Apache's vhosting working for multiple websites.  This is for about 10 domains.  Each 
of these domains need their own mail accounts.  I do not want these mail account to be actual 
system accounts. As there will only be may be 24-30 email addresses in total I would like to 
avoid the additional complexity of integrating a separate database such as MySQL etc into the 
mix.  Users should be able to send mail to this box via smtp and receive via pop3...just normal 
mail stuff.

Functionality desired:
- A webmail interface... remote administration(ie add/delete new accounts for a given domain) 
would be nice but not mandatory...not sure if this is incompatible with my desire to avoid 
integrating a 3rd party database into the scheme.

One of my major questions is whether adding this kind of functionality to my box means 
ripping out the default Sendmail install.  If possible I would prefer to leave the default Sendmail 
install in place and have it continue to handle system messages for the localhost as usual.  If I 
need extra software to provide my required functionality I would prefer to keep it isolated to only 
performing for these virtual mailboxes.   Is this possible or is either all or nothing, use only 
Sendmail or use only something else?  If it is possible, what ports may be useful to explore 
furthur?