[Dovecot] about acl

[Yavuz Maşlak]

I use dovecot, vpopmail.

I wish to restrict a mail user's login to its mailbox according to its ip 
address.
I don't wish other users to be affected .
Namely, for instance,  abc at domain.com can login to its mailbox from a few
specific ip addresses but the same user will not able to login from
anywhere.
Meanwhile normally other mail users can login to their mailboxes from all ip 
addresses.
How can I do that with dovecot acl ?
any advice.