Zimbra Forums

I use Zimbra with unseen.is webmail. Today, no emails (to individual recipients) will send. This has happened before but somehow resolved but today will not.
The message I get is:
Message not sent; one or more addresses were not accepted.
These are frequent and long-time contacts with different email providers.
Does anyone have any idea how this can be fixed?
Thanks,
Ellis

I'm afraid these forums are nothing to do with that company nor can we provide any support for them, you need to contact their administrator directly.

Thank you Bill, but I cannot reach them. As they are running on the Zimbra platform I thought this might be ok to ask here.

We are simply not allowed (by Zimbra) to provide support for any other organisation that provides a service using Zimbra. If there was a simple answer then you'd get one here but as we have no access to their service or log files (as well as the restriction I've mentioned) then we couldn't possibly know what's happening on their servers.