Email from Website trapped in webdomain

We are using IIS with smtp for customers to send comments to our customer service department, however the mail is getting trapped in the webserver and is not going to the mailbox assigned. In other words, you fill out a form on mydomain.com and when you hit submit it should be sent to customerservice@mydomain.com, but it is getting trapped in the webserver in a bad mail file rather than being sent to the correct mailbox which resides in the email server. My thought is that DNS forces the mailback to the webserver rather than permiting ti to go out to the email server. Any thoughts?
thanks