Configure mail server

We have a IMPORTANT problem with our mail server, we use IMAP secured through TLS and SMTP secured. WE use postfix, spamassassin, clamav and amavisd. The [login to view URL] MX health checker say we have no problem with it. But there are a lot of LOST emails ; we send mail to addresses, but recipients are never receiving our mails. Internal mails are lost very often, I don't know what to do, else pay someone to correct all these problems. Other servers from our webhosting servers also use mails, but they can't send mails anymore. Please, engineers only with real skills in mail servers with postfix/debian. URGENT.