Strange Postfix message

[Curtis Vaughan <curtis@npc-usa.com>]

If I send a message to more than one recipient on my postfix server, I 
get a message back stating that each recipient does not exist. 
Specifically:

Subject: Undelivered Mail Returned to Sender

> Subject: Undelivered Mail Returned to Sender
>
> This is the Postfix program at host pacifica.vostok1.com.
>
> I'm sorry to have to inform you that the message returned
> below could not be delivered to one or more destinations.
>
> 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 message returned below.
>
> 			The Postfix program
>
> <user1@vostok1.com>: user unknown. Command output: : Mailbox
> does not
>     exist _
>
> <user2@vostok1.com>: user unknown. Command output: : Mailbox
> does not
>     exist _

However, they both receive the message.
If i were to send them each a separate message, however, everything 
would go through fine.

Why?