[FIX] portal: portal send mail with right email address

Go on the portal with a portal user
Drop a message in the chatter

The message is logged in the chatter from the right partner
Before this commit however, the email that was sent had
its address from of the OdooBot

After this commit, the mail is sent with the partner's email from

OPW 2074154

closes odoo/odoo#38507

Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>