I checked, it work perfect.
    

OK I take your words for that.

The only thing I can say is that the error you mentionned saying it
cannot write to the directory /var/mail/client-mqueue or something
like that is typically a problem with the configuration of sendmail.

Sendmail will be able to receive email submited by root, but not by
other users.

One solution (but it opens a security issue) is to change sendmail to
be suid root.

Olivier