Question : Lots of Mail Undelivered

Hi.
I have a web application running on IIS - that sends mail to my mail server running merak mail for delivery.

Both the web server & mail server are behind a firewall so have 192.168...IP's

The mail server has it's own domain - lets call it myapplicationmail.com mapped to its public IP

The application sends mail on behalf of the application user - sent to their clients. All mail is essentially from our clients email to their clients - stuff like payment receipts, booking reminders and so on. So the application will send a message like

FROM : info@applicationsubscriber.com
TO : [email protected]
SUBJECT : Booking Reminder

This mail is relayed from the application to the mail server.  Lots & lots of messages are undelivered in the mail queue.

I set up an SPF record for the mailserver domain that allows the internal IP range of the network that include the webserver.

What I can I do to get these mails accepted by the recieving mail servers?  I'm guessing I would have to ask my clients to add myapplicationmail.com to an SPF record for their domain so my mail server is a trusted sender of their mail. That step would be a holy nightmare - about 1% of my customers would understand what that meant.

Is there something I am missing - an easier way to ensure my mail server doesn't look like a spammer so mails at least dont get rejected by the recieving mail server.

Answer : Lots of Mail Undelivered

Are you saying the webserver is having trouble sending the mail to the mail server to relay, or the mail server itself is having trouble sending the messages to the external recipients?

I would check the usual suspects:
- proper, valid, non-dynamic-looking (host1-23-34-45.host.com) reverse DNS
- corresponding forward dns
- HELO name matching reverse DNS

90% of the time one of those things is the problem. I wouldn't worry about SPF, it's more of a way of telling who isn't legit rather than who is legit.