[Dovecot] users can not login

[Daminto Lie]

Hi,

I was wondering if I could get some help here. Recently, I decided I  want to 
move from Windows XP Pro OS to a Linux OS such as Ubuntu. I am  new to Linux OS 
but willing to learn. I have installed Ubuntu Server  10.04, Apache2, MySQL5, 
PHPMyAdmin, Postfix, DoveCot and Squirrelmail on  my server. I have checked them 
all and they are all up and running at  the moment. Now, what I am trying to do 
here is to add a couple of users  and see whether they can login to 
squirrelmail. The result is none of  those two users are able to login. I went 
to check the mail.log and the  error message is as follows
Ubuntu-MailServer dovecot: imap-login: Aborted login (auth failed, 1  attempts): 
user=<david.keith>, method=PLAIN, rip=127.0.0.1,  lip=127.0.0.1, secured 


My silly question is I do not quite understand what the above error  means. How 
do i enable users to login and test to see if they can send  and receive emails. 
Users can login via squirrelmail.

Any help would be greatly appreciated.