Linux Mail Server Setup and Howto Guide

I am new to linux though i have setup a few basic email servers. i had setup centos5, sendmail and squirrelmail. all was fine until i tempered with dovecot, what i was trying to do was to setup imap server so i can keep copies of emails on the server. and the last command i did is:  makemap hash /etc/mail/access.db </etc/mail/access

then  had a power cut. now i can send emails to users on the server no errors or returned emails but  they cant download from thier outlook clients. the mails are not visible either from squirrelmal (webmail) i can see the emails if i do webmin and check user mailboxes. outlook is not reporting any errors and the mailogs are very clean. I copied back the untempered dovecot , restart dovecot, sendmail…no errors

What can i check.

Ultimately when i get going again i want to setup two doamins and be able to select what users use whic domain even from webmail…?? hope i make sense.