I have been using Ubuntu for my server. Its a personal server so I am able to get away with just 512m droplet.

If using Ubuntu 13.10 you will need the 3.0.5.4 RC1 of ispconfig. It officially supports 13.10 with apache 2.4. You can tweak the current release version of ispconfig to work if you use nginx.

I have ispconfig up and runing. I wand suggestions for mailservers only

Dear Meletis Flevarakis,

You can consider using postfix as your mailserver:

Best Wishes,

Geneva Sibanda
http://magwinya.co.za

Guess I am not sure what you are looking for.

Most Ispconfig setups (Perfect server) use postfix as mta and dovecot or courier as imap/pop3 daemon and Squirrelmail as webmail client. See http://www.ispconfig.org/page/en/documentation.html for anyone else interested in setups. I am guessing the OP is already past this point.

They have a plugin for Roundcube that ingrates into ispconfig database. It as little extra work but not hard. https://github.com/w2c/ispconfig3_roundcube Same for squirrelmail

I personal use Horde as my webmail client. Pain to setup!

Hope that helps.