message not deliverd

["postoffice@xxxxxxxxxxxx" <postoffice@xxxxxxxxxxxx>]

Hi all,

today we are experiencing a ne problem, some people is calling to tell us
that mail sent to us are Bounced to sender.

They receive an error message (from their server I guess) like this:

"555 Invalid MAIL extension"

Is this a Post Office error message or what?

Please Help!
Thanks


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