interesting email issue

[viperiii]

created an email user "viper_iii@allkinds.ods.org"

The email exists and I am able to use webmail and send mail from the account but replying to the email the server says

Quote:This is the mail system at host allkinds.ods.org.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

The mail system

<viper_iii@allkinds.ods.org>: unknown user: "viper_iii"



Action: failed
Status: 5.1.1
Diagnostic-Code: X-Postfix; unknown user: "viper_iii"

it seems that postfix has an issue with "_" (the underscore)

anyone else confirm this and ideas on how to make postfix happy about this? (fix?)