Troubleshoot IIS SMTP mail domain settings so that it's recognised a legitimate sender

We are sending email through a web application using IIS 6.0 and SMTP. Just to explain what's happening we'll use ' mydomain ' in place of our real domain name. Firstly, the default domain in the properties section of HOST1>>Virtual Server#1 on the Access Tab under Authentication is [login to view URL] dot com. If we click on the Domains folder under Virtual Server#1, the name of the domain is mydomainmail dot com. The emails are being sent from info at mydomain dot com . We are fowarding all emails from info at mydomain dot com to a gmail account, let's say forwardingmydomainemail at gmail dot com . These emails are downloaded to local application that processes all incoming emails and stores them on a CRM system. We noticed that undelivered emails were going into the spam folder on the gmail account but we weren't concerned about that. Now we want to process the emails that were undelivered to clean our database. However, because the undelivered emails are going to spam, we are not able to download them to our applications for processing the emails. Here is the message that we see on gmail:- "postmaster at mydomainmail dot com via outbound-myemailprovider dot com to info Why is this message in Spam? It seems to be a fake "bounce" reply to a message that you didn't actually send. " We already tried to change the domain for sending the emails to mydomain dot com instead of mydomainmail dot com so that the bounce emails will come from postmaster at mydomain dot com . mydomain dot com has an A record pointing to the IP address of this server and it has a reverse lookup for that ip address also. However when we did this, the undelivered mails are not being returned to info at mydomain dot com. We think this may be because the server thinks it's hosting emails for mydomain dot com and so the returned emails are being delivered internally on the server. We are looking for some help from someone who has come across this type of thing before and either tell us how to configure the server so that it doesn't think it's hosting the mydomain dot com email. If this can't be done the only other solution we can think of is to register a domain called mydomainmail dot com and add an A record and/or TXT record etc for the IP address on this server so that when gmail receives an email from postmaster at mydomainmail dot com with an ' Delivery Status Notification (Failure)' Subject that it will recognise it as a legitimate email and not a fake bounce. If you are interested in providing a quote to advise us how to fix this issue please include the word 'Bounce' in your reply to show you have read the description.