[Dovecot] virtual domain

[Barry Kwok]

we are currently using vm-pop3d to implement virtual domains. But due to some locking problem in vm-pop3d,   we want to test the possibility to change to dovecot.
Currently, in vm-pop3d, the passwd file is located in
/etc/virtual/domain1.com/passwd
/etc/virtual/domain2.com/passwd

and the passwd format is:
user: md5 encrpted password

and the email is store in 
/var/spool/virtual/domain1.com/user
/var/spool/virtual/domain2.com/user

and we login as user at domain1.com, user at domain2.com . 
We want to maintain this format as login id and the password format. 
What should be changed in dovecot.conf? Thanks.