Hi,

A custom email server in odoo is particularly useful when you want to customize your email infrastructure or use a dedicated email server for your Odoo instance.

1. Go to Settings-> General settings-> Discuss -> Enable Use Custom Email Servers
We may enable the field 'Custom Email Servers' here, which will bring up another option below where we can enter the custom domain. After creating the custom domain or alias domain, 

2. The next is to create a record in the model ‘mail.alias’ from Settings > Technical > Email > Aliases.
For more details, refer to the Odoo 16 book and blog:https://www.cybrosys.com/odoo/odoo-books/odoo-16-development/emails/email-alias/#:~:text=We%20may%20enable%20the%20field,%3E%20Technical%20%3E%20Email%20%3E%20Aliases.

https://www.cybrosys.com/blog/how-to-configure-a-mail-server-in-odoo-16

Hope it helps