The easiest one I've ever installed is postfix with dovecot. There is a dovecot-postfix package, but it also looks like the Ubuntu server team has packaged this as mail-stack-delivery :
Description: mail server delivery agent stack provided by Ubuntu server team
Ubuntu's mail stack provides fully operational delivery with
safe defaults and additional options. Out of the box it supports IMAP,
POP3 and SMTP services with SASL authentication and Maildir as default
Looking in the Software Center, I see that dovecot-postfix is present as a meta-pacakge (but I don't know if it will be called that a release or two from now). But that is what I'd recommend -- it's straightforward and secure.
From the command line on my 10.10 desktop, I installed this via apt-get :
sudo apt-get install dovecot-postfix
Give it a shot if you like, and if you have questions, just start a new post.
Here's a oldish (2009) Ubuntu Server Blog post that gives some of the details on the dovecot-postfix configuration.
I'm trying to find out more information on the current status of Ubuntu-centric mailserver projects. Right now I'm not finding much more than a May 2010 wiki post about making anti-spam/virus integration easier.