[Dovecot] virtual domain

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