Sometimes, permissions can become corrupt for SmarterMail which prevents attachments from being displayed in webmail.  This article goes over fixing this. 

Steps
An error like this can sometimes be seen when trying to get the attachment:
XML Parsing Error: no element found

Before you proceed, do a SmarterMail self-diagnostic (also known as checkup page), this will help you identify the problem.

1. Log in to SmarterMail as the system administrator.
2. Click Settings (top left).
3. Under Activation click SmarterMail Self Diagnostic.

This will launch a separate window showing you the problem.

For permissions:

• C:\Program Files (x86)\SmarterTools\SmarterMail\MRS\MailProcessing
• C:\Program Files (x86)\SmarterTools\SmarterMail\MRS\App_Data

Please apply the permission (read and write) to your user, network service, IUSR. Then, please run again Self Diagnostic test.