Inbound emails from GMail undeliverable only to Fortimail

There are several gmail accounts that send emails to our organization but just won't come in the inbox. The sender then gets a mail delivery failure error on Gmail. We did some tests from other gamil accounts and the mails get in without a problem. Its like as if Fortimail has blacklisted those accounts. Diagnostic-Code: smtp; 554 5.3.0 connection not allowed