This has been happening periodically...one of our end-users will send an email to an external recipient. They can have an email conversation. Then, without explanation, the end-user starts receiving messages like the one below when they try to send another message to the external address a day or two later. We've verified that the address is correct. How do we keep this from happening?

HERE'S THE MESSAGE:

This is the Spam Firewall at mail.hendricks.k12.in.us.

I'm sorry to inform you that the message below could not be delivered.
When delivery was attempted, the following error was returned.


<jbarxxxxxx@marian.xxx>: host sc04-311.marian.abc[8.15.7.xxx] said: 550
Hostname/Domain name not valid or not available. (in reply to RCPT TO
command)

Edit: Email addresses, IP's and domain names obfuscated for privacy and to foil spambots. ~ Animal

Hello waggonera,

When a Host name/Domain name is rejected, this indicates that the sender's public IP address has been added to the Barracuda's customizable blocklist.
Having never used Barracuda, how about trying their Support Center.