Can't receive mails in a domain

[tspau]

hello!

something strange happens to me.

i have several clients and sites. one of the clients has two sites, two dns zones and two mail domains. both mail domains have the same settings. with the mailboxes of one of the domains, it's all ok, i can send and receive mails.
instead, with the other one, i can create mailboxes, and login with roundcube or thunderbird, but i don't recevie the new user mail, also i can send mails with this accounts, but if i try to send a mail to this accounts, i receive something like this:

This is the mail system at host XXXXXX.com.

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

<pau@XXXXXX.com>: unknown user: "pau"

any ideas, please?