Hi JacoBosch
The SMTP is completed removed on 20.0 version
But it was marked as removal on 17 version of BC
So you have to add email in email accounts page or else it will give you warning.
But can you tell me how is your Business Central version is still on 18. Microsoft forcefully upgrades the BC version at some point of time.
If my answer was helpful to you, please verify it so that other users know it worked. Thank you very much