If you check correspondent email message entity, "en_email_message" record, is there any error message reported by the SMTP relay?
Does DG SMTP email address look correct in the generated email message?
Could you try to copy DG SMTP email from the record and send test email?
If you can answer "yes" on the questions above, most likely, account used to send emails from the ClearQuest is not authorized to send to the distribution list. Also, you might also need to talk to your mail administrator: you have log of the email message delivered to the SMTP relay (en_email_message record), and it is lost somewhere after that.