Is the server in question running ISPConfig? I'll assume that it is until you state otherwise.
The problem is most likely with the following line in main.cf
mydestination = $myhostname, localhost.$mydomain, $mydomain
You might try changing it to:
mydestination = localhost, localhost.localdomain
Do you have the following error in your logs?
postfix/trivial-rewrite[XXXX]: warning: do not list domain mydomain.com in BOTH mydestination and virtual_mailbox_domains
See my post here for details:
Also, if ISPConfig is installed, what value are you using (in main.cf
Finally, is this value accurate? Are you really using the "mail" subdomain?
myhostname = mail.domain.com