We are running a new exchange 2010 SP2 installation. Sending attachments in big size is no problem within the domain, but every external mail with an attachment over 500 kb is just lost. No error message to the sender, no message in queue, just nothing. Tried every size settings in the hub transport, receive connectors and the mailboxes. It's just a simple network with a Zyxel USG20 firewall with deactivated content filters. Tried also to deactivate the firewall and antivirus. Any idea?