One or more servers can't retrieve the outgoing email credentials (SharePoint Server 2019)
APPLIES TO: 
2013 2016 
2019 Subscription Edition SharePoint in Microsoft 365
Rule Name: One or more servers can't retrieve the outgoing email credentials.
Summary: At least one web application is configured to use authentication when sending email. There are one or more servers in this farm that can't retrieve the credentials used to authenticate to the outgoing email server. Without credentials, these servers can only send email anonymously.
Cause: The application credential key wasn't found on these servers or they don't have the same application credential key originally used to store the SMTP password. Every server in the farm must have an application credential key to store and retrieve the SMTP password. The application credential key must be identical on each server.
Resolution: Use the Set-SPApplicationCredentialKey cmdlet on each failing server to set the application credential key. If the current SMTP password was stored using a different application credential key, you must set the new application credential key on every server in the farm and then save the SMTP credentials again.