You may see the following error in your Windows Application Log after making modifications to your SharePoint Farm especially surrounding Incoming Mail Server Settings.
This could be due in part to the following:
- Access level of these Local Groups where SMTP services is installed. The two accounts are WSS_ADMIN_WPG and WSS_WPG
- Whether or not the “Drop” folder is in the place SharePoint expects it to be in based on where your INETPUB folder is installed
The fix i have found is to
- Give WSS_ADMIN_WPG and WSS_WPG full access to the DROP folder under Mail Root under InetPub
- Select Advanced in CA under Enable Incoming Email and specify the E-Mail Drop folder explicitly
Hope this helps!