Hello,

This issue occurs because, in Odoo 19, the Snailmail module requires certain fields to be filled in for each record. When you open Settings → Technical → Email → Snailmail, the system may display a “MISSING REQUIRED FIELDS” error if any of the following fields are empty:

• partner_id

• state

• country
  check also if there is any other required fields.

These fields are mandatory for the module to function correctly. To resolve the issue, you need to open each Snailmail record and populate all required fields before saving. Once all mandatory fields are filled, the error should disappear.
If the problem persists, it could indicate a misconfiguration in the module installation, and you may need to reinstall or update the Snailmail module to ensure all required fields are correctly defined.