3341119 - No email is sent from IAS when using custom mail server

User does not receive any emails from IAS. When reviewing the logs, the following error can be seen (mail subject is an example):

"yyyy-mm-ddThh.mm.ss.000Z timestamp="yyyy-mm-ddThh.mm.ss.000Z", ipAddress="<ipadress>", severity="INFO", location="umtrace", crtAccount="<TenantId>", authenticatedSubject="anonymous", 
correlationId="<ID>", state="failed", action="sendMail", objectType="user", objectId="xxx@companyname.com", 
message="Sending the email to the following server failed :<mail server>", category="audit.configuration", mailSubject="How to reset your password", 
currentPoolSize="300", currentActiveThreads="1"

SAP Cloud Identity Services

custom mail server, Email could not be sent to, Sending the email to the following server failed, smtp.office365.com:587, Authentication unsuccessful, smtp.office365.com, user credentials were incorrect , KBA , BC-IAM-IDS , Identity Authentication Service , Problem