mail disapearing form server

We run a mail server from the office - this is IMAP and mail is directed to the server via MX records pointing to the office. lately mail seams to be disapearing randomly from mail boxes.

the mesage gets delivered and the disapears a few seconds later - its not in trash nor in junk folders?

some messages dont arrive at all - collegues copied in on conversatons do recive the messages though?

its only recently started after a year of perfect function. the mail boxes are not large and no rules are in place to block or filter mail.

some users have there IMAP accounts on phones and a few macs but the missing messages can not be found on any of them.

any ideas?