Email error messages when sending to other staff

Some staff have reported that mail they sent to other staff this morning has been returned to them with an error message 550. After extensive investigation, here's my best guess of what is happening:

When Postini sees a spam attack, it blocks spam messages and sends back "ERROR 550 Mailbox not available" to the spam source to fool it into thinking that the mailbox doesn't exist.

Postini found a spam attack that originated from a Google server. It put a block in place. Email messages from our staff to other staff which happened to originate from the same IP address as the spam attack were blocked, and the 550 error was sent back to the original sender.

That's my guess. To test it, I changed the bounceback message in Postini to 571 "Spam source blocked." (The block is no longer in place, but could start up again.)

If you get a bounceback error message when you try to send a message to other staff, please do the following:

1. Click on the down arrow in the top right corner of the message and select "Show Original"
2. Copy the resulting text into an email message.
3. Send it to me.
4. If it bounces back to you, please fax it to me (x1309) or print it to our printer (it's called DSO_9)

Thanks!