Hi,
Based on the error message, Exchange is looking for the certificate for the name "journal.xxx.partnercompany.net" which is the smarthost in the SendConnector. Please check if the certificate for "journal.xxx.partnercompany.net" is installed on the certificate personal store on the exchange server. If yes, check the validity and If not, install it.
Get the list of certificates using Get-ExchangeCertificate
Get the thumbprint and enable the SMTP service using the below command
Enable-ExchangeCertificate -Thumbprint <String> -Services SMTP