Mails can't be delivered to inside managed domains

[pioup]

Hi,

Since my brand new installation on centos5, mails can't be delivered inside the server.

Customer can't send mails trough webmail or email clients to same server managed domains, smtp server refuse to do so but it allows to send mail to external domains.

error :
Could not send message to SMTP server. Check you have access to send messages via the server and that all To/CC/BCC addresses are valid\nError: SMTP Server rejected email. Returned:550 sorry, no mailbox here by that name. (#5.7.17)

PHP & Perl mail functions not working with inside managed domains neither

/usr/sbin/sendmail -f [email protected] [email protected] //OK
/usr/sbin/sendmail -f [email protected] [email protected] //KO

Where can I check/repair Mail routing ?

Many thanks.