I reviewed this Wiki
Outgoing SMTP Authentication - Zimbra :: Wiki
I tried is on my VM and outbound mail is stuck in the Active queue.
I get a bounce back of:
host outgoing.verizon.net[188.8.131.52] said: 550
5.7.1 Authentication Required (in reply to MAIL FROM command)
I then noticed that the Wiki applies to the following ZCS versions 4.5,5 and 6.
any help would be great!