1- When running your application in a seperate Application pool, Make sure the identity of this App Pool has the appropriate access to SMTP obeject. For Example if the App pool uses "NETWORK SERVICE" Identity, "NETWORK SERVICE" must appear in SMPT access list. To check this:
a) START > Administrative Tools > Internet Information Services (IIS) Manager
b) Right click on the Default SMTP Virtual Server and select Properties.
c) Click the [Security] tab.
2- Make sure that the App Pool Identity (e.g "NETWORK SERVICE") has the apropriate permission to :
This is the root folder for SMTP email. To adjust the permission for this folder add the CREATOR OWNER with full access and your App pool user with read/write(Only write) access. This ensures that the user can write his emails to this folder but they can only control their own mail.