Hi, you have a mix between old and new external gateway setup.

From 8.5 2018-04-20,


External Email gateway is starting up automatically with Standard ERP server


What is needed:
1. External Email to be active in System >> Settings >> Configuration >> CRM tile
2. Use External Gateway to be active in System >> Settings >> Optional Features

When activated, new folder will be created in a server folder: EmailGateway

There is no need any more to use External Gateway as a slave system that occupies one user slot.Automatic external gateway only runs, if Gateway user field is empty. This can happen if you import backup with gate user setup, or update from earlier versions.

Email and conferences>>settings>>email SMTP server :
Start Gateway Automatically should be switched off, Gateway user field should be emptied, Gateway password should be empty.

If they are filled in, they will override automatic Gateway.

To see if you are running the new GATE, either check System>>settings>>Active users
You will see GATE as user - this Asterix ( * ) means you have the new 8.5 gate running.
If you see GATE - you are running the old way.

Other method to check, open Server folder, EmailGateway folder, and open hansa.log. There you should find :*GATE* Running as External Mail Gateway


Update:
Database must be "live database" so that it doesn't steal emails from live if it is not