[Mailman-Users] Unknown user when posting to a list?

[Takahiro_Horie at nidek.com]

Hello,

I created a list and succesfully subscribed several e-mails (the welcome 
e-mail was received by them).
However, if someone tries to send to the list (test at thorie.com), they get 
an error message from the server saying that the user 'test' was not 
found.

Why is this?

Am I supposed to create a user account called 'test'?

Thanks in advance!
--Takahiro