3560862 - Users are not receiving emails from IAS: Unexpected character ('<' (code 60))

• When the default mail server is used, emails are sent successfully.

• A custom mail server is configured in IAS and when using the custom mail server, the following error is observed:
  
  Error occurred in IdsMailingService while trying to send mail to xxx@sap.com. IDS Mailing Exception: Email could not be sent to xxx@sap.com with subject Activate Your Account for User Profile. Caused by: com.fasterxml.jackson.core.JsonParseException: Unexpected character ('<' (code 60)): expected a valid value (JSON String, Number, Array, Object or token 'null', 'true' or 'false')_ at [Source: (StringReader); line: 3, column: 2] Caused by: Unexpected character ('<' (code 60)): expected a valid value (JSON String, Number, Array, Object or token 'null', 'true' or 'false')_ at [Source: (StringReader); line: 3, column: 2] Correlation ID: XXXXXXXXX 

SAP Cloud Identity Services

IdsMailingService, Email could not be sent, Unexpected character, com.fasterxml.jackson.core.JsonParseException, Custom Mail server, default template, , KBA , BC-IAM-IDS , Identity Authentication Service , Problem