Some mail being dropped.

Hello all

My finance software (SQL-Ledger) is supposed to email invoices directly to clients. I've set up Sendmail for send only and defined the 'Smart Host' and 'most' of my customers are getting the emails but about 10% are simply not receiving them. I'm trying to track down and figure out why not an where they are getting either lost or dropped.

I think I may have messed up my server naming structure so I'll start by laying out what I've got.

I have a website that is hosted and has it's own fixed IP, 'out on the www'

My website address is and resolves to a static IP.

Inside my business I have a rack of HP Proliants:

The 320 is loaded with Monowall it is the firewall, DNS and DHCP server.

There is a 360 with FreeBSD 9.3, Postgresql, Apache, Perl, Sendmail and SQL-Ledger for finance. It serves the interior of the business only.

On this 'finance' server:

I've named the server inside the firewall but it's IP is a address that is a fixed address I set up in the DNS section of the monowall server.

I'm using sendmail to deliver the mail and I've set the "Smarthost" to that of my ISP. (I've been told that I must use the ISPs outgoing mail server and not the one from my hosted website.)

I edited the .mc file to:
define ('SMART_HOST', '')

and I changed the rc.conf to:

The emails being sent by have the sender address as

I'm wondering if the fact that the mail is coming from an internal server, that doesn't have the same IP as the DNS that the name resolves to 'outside', is causing some of the better spam filters to reject it.

Is my naming structure incorrect?