I'm encountering exactly the same problem as described here but in Odoo 16 (Online Custom), also with Oracle OCI smtp mail infrastructure, suspecting also the very long user names Oracle is generating but this is nothing that can be controlled

After encountering challenges with Oracle Mail Delivery and limitations in changing SMTP credentials directly, I explored using SendGrid as an alternative SMTP provider. Despite facing constraints with both Oracle and SendGrid, the workaround that worked for me was adapting to the SMTP relay of SendGrid. This change successfully resolved the email delivery issues I experienced with Odoo.

If you're facing similar hurdles with Oracle's SMTP configuration, consider exploring alternative providers like SendGrid. I suspect the issue with Oracle is their super long credentials. SMTP Username is more than 100chars long in Oracle Email Delivery and in SendGrid it is just 6 char long. hh

Hi,

Please go through the below screenshot and verify all the provided details are in correct manner.

In the configuration form, you need to provide the following information:

* Name: Give your outgoing mail server a descriptive name.
* Authenticate With: User Name
* SMTP Server: smtp.gmail.com
* SMTP Port: The port of your SMTP server, here it is 465
* Connection Encryption: SSL/TLS
* Username: Your Email Id
* Password: Provide the password of your Email ID
* Priority: A lower numerical value indicates a higher priority.

Please refer the below blog too
https://www.cybrosys.com/blog/how-to-configure-outgoing-and-incoming-mail-servers-in-odoo-17

Hope it helps