[Dovecot] Subscriptions - delivering to different folders

[bichumo]

Hi,

I am using Postfix and Dovecot. I have set up backup_mailbox at domain.tld, 
which gets mail copies from user1 at domain.tld user2 at domain.tld 
user3 at domain.tld, the copy option is set in postfix recipient_bcc_maps = 
path_to_file_recipient_bcc. In recipient_bcc file I have:

user1 at domain.tld backup_mailbox at domain.tld
user2 at domain.tld backup_mailbox at domain.tld
user3 at domain.tld backup_mailbox at domain.tld

Postfix does the job and copies user1, user2, user3 emails to 
backup_mailbox. But I also want to filter mails in backup_mailbox and to 
put user1 mails to USER1 folder in backup_mailbox, user2 mails to USER2 
folder in backup_mailbox and so on. How this is possible? I made an 
entry in subscriptions file of user1, user2, user3. Now I have folders 
.user1, .user2, user3 in backup_mailbox folder and I can also see these 
folders through webmail client (squirrelmail), but how do I put email 
copy which comes from user1 at domain.tld to backup_mailbox at domain.tld to 
go not to main folder of backup_mailbox but to USER1 folder in 
backup_mailbox folder.

Thanks for any suggestions.