Issue with mails after migrating to IIS7

Hi

We recently migrated our applications to IIS7. Everything works fine except for the mail functionality. After Migration, we are not able to send mails

We noticed that, if we add the below code, the mail functions as usual.

System.Web.Mail.SmtpMail.SmtpServer ="mailhost";  

Kindly, let us know why we are getting this issue with mail functionality and how we can resolve this issue without adding the above mentioned code.

Thanks

Rahul