This usually happens if some custom permissions are applied to the Webppliance machine. By default the Mailmaxweb service requires the IUSR of the local machine read access to the C drive only.
In order to fix this issue please go to the Properties of "C" drive over there go to security under security you will find that the user IUSR_machine name does not have any rights. You will have to add the user IUSR_machine name to the list then click on
Advanced --> View Edit --> Apply onto this folder only
Select the following permissions
List Folder / Read Data
Read Extended Attributes
Click on ok and apply these permission.
For detailed information you can also have a look at the attachment as to how permissions are set.
Now the webmail should be able to send mails with attachments without any problems.