I now understand that you have Odoo to submit Emails with random SEND/FROM addresses to your Mail-server; where your Mail-server’s policy is set to not relay requests with random SEND/FROM addresses.
Therefore you would you like your Mail-server to relay all E-mails with any SEND/FROM address that ends with your domain name.
We use an Exchange mail-server and have set-up a second receive connector for that purpose. The second receive connector will relay any mail from our Odoo server. Our policies are that we use secure SMTP communication, the SEND address must be within the domain name; and the mail may only come from our authenticated Odoo server. Which is fully under our own control.
An article about how to set this up for Exchange is here: https://practical365.com/exchange-server/exchange-2016-smtp-relay-connector/
If you are using a third party SMTP service, then you may have to ask them to set this up for you. Since this could open the door for SPAM misuse, they may not be too happy to do so without some security guarantees. A TLS certificate would be required.
EDIT FOR CLARITY: i have no problem with replay addresses. The problem is that my SMTP server only allows to send emails FROM addresses which have been verified: whenever i want to send an email FROM new_address@mydomain.com i would recieve a verification email at new_address@mydomain.com with a confirmation link to confirm that i am the owner of this email. When i send an news email from Odoo, it sends it FROM random email addresses and i can not verify all of them before sending a new email. Therefore my SMTP server rejects the outgoing email because the FROM field in the email is not a validated address.
I think this post is about a similiar issue but none of the answers in the post worked for me.
This post: https://www.odoo.com/forum/help-1/question/error-sending-email-71602
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