Customer upgraded to 8.4 (same for 8.5) Old persons can send emails, new persons setup after upgrade cannot. The person can send emails if given the email address of another user who can send. SO it would appear to be new email addresses or mailboxes that have the issue.

Server log gives message:

/THREAD(4) 550 5.7.1 Client does not have permissions to send as this sender"

Email remains in queue with sent marker 0