Authorization of users in Postfix using the mailbox address?

Question

postfix

There is a Server on Debian 7 x64.
Virtualmin and its standard dovecot+postfix set with SASL/PAM authentication are installed:
a user is created in the system for each mail account.
Sending and receiving emails works without problems.
But there is one unpleasant nuance.
Linux usernames cannot contain the "@" symbol, so the system replaces it with "-" for mail users and cuts off the ending, for example, the [email protected] mail account corresponds to the user-domain system user.
When connecting via Webmail Roundcube (which is installed on the same server) - the user can use both the user-domain username and the [email protected] mail address for authorization.
However, when connecting remotely, there are nuances.
Authorization for receiving emails (regardless if it is imap or pop3 protocol) occurs as in the case of Roundcube in both cases: using user-domain username or [email protected] email address.
Connection to SMTP occurs only if you use a username, if you specify a mailing address, then the mail client receives an authorization error response from the server: incorrect username or password.
This server behavior causes inconvenience, the user must know both his address and username, which is an additional headache for those who are not strong in IT.
Question:
How can I configure dovecot/postfix to be able to connect to SMTP using both username and email address?
I found a recommendation on the Internet to set the parameter in /etc/dovecot/dovecot.conf:
auth_username_format=%n
But this only partially solves the issue, connecting using both methods only works for the domain that was first registered in the system, it does not affect the rest.
Thank you!