So this is a single system user with single email, there are a number of system users and email accounts, I don't need to use SQL or LDAP or another password file. I can write python script to analyze the logins and the mail folders including quota and send to socket file , But how the formats as dict ? How the user input ( email/password ) send to that unix socket ? I mean the format The documentation doesn't say anything about it.

https://doc.dovecot.org/configuration_manual/authentication/dict/ There is an example script done in perl at the bottom of the page. Maybe it can give you hints as to the format needed for a python script.

On the same page it also says: When given a proxy URL the Dict backend speaks a simple protocol over a UNIX socket. The protocol is defined in src/lib-dict/dict-client.h https://github.com/dovecot/core/blob/master/src/lib-dict/dict-client.h