Emails being bounced back to senders on one specific account

[ANAX BT]

Sent an email to an active Zimbra email account on Friday.
Saturday I got back this 'Undeliverable' email from the email server:

"Delivery has failed to these recipients or groups:
Eric *** (eric*@****.net)
Your message couldn't be delivered. Despite repeated attempts to contact the recipient's email system it didn't respond.
Contact the recipient by some other means (by phone, for example) and ask them to tell their email admin that it appears that their email system isn't accepting connection requests from your email system. Give them the error details shown below. It's likely that the recipient's email admin is the only one who can fix this problem.
For more information and tips to fix this issue see this article: https://go.microsoft.com/fwlink/?LinkId=389361."

I see the account in Zimbra Administration (8.8.15_GA_4272.FOSS)
The Eric email is listed as Active and last logged into earlier this month.
I can log into the email account and send emails successfully but the account cannot receive emails.

Any ideas?

Thanks in advance.

[phoenix]

Please post in the correct forum if you have a problem, the Zimbra Feedback forum is not for installation problem reporting. I'll move this to the correct forum.