Hi, you need to configure the catchall@ mail address. This hast do be as reply address in your out-mails. If someone is responding with that reply (catchall@), Odoo is routing internally the mail to the correct thread.

see here config catchall@: https://www.odoo.com/documentation/user/12.0/discuss/email_servers.html

Hi, this is the way it is supposed to work, you shall get the response email in the chatter from where you sent the email. Moreover, Odoo will generate an email to your user telling you received an answer. You just have to click on the email to arrive on the right screen in Odoo.

If it doesn't work, it's going to be related to your configuration...