You are using Symantec Mail Security for Microsoft Exchange. You have configured Symantec Mail Security for Microsoft Exchange to tag subject lines of emails that violate specific rules. When you open an email that has been tagged in Outlook, the email has strange characters in it.
 

This problem occurs when the subject line gets encoded. To avoid this problem you must disable subject line encoding. This solution works with Symantec Mail Security 5.0.3 for Microsoft Exchange and later versions.

To disable subject line encoding

1. On your Exchange server, open Regedit.
2. In the Regedit window, browse to the following registry key:
   
   HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\SMSMSE\<version>\Server\Components\SMTP
    
3. On the toolbar, click Edit > New > DWORD Value.
   This action creates a new DWORD value and highlights the name of the value.
4. For the name of the new value, type EncodeSubjectLine.
5. Double-click the new EncodeSubjectLine value.
6. In the Edit DWORD Value dialog box, in the Value data box, type 0.
7. Click OK.
8. Close the Regedit window.
9. Restart the Symantec Mail Security for Microsoft Exchange service.

Technical Information
When Symantec Mail Security for Microsoft Exchange tags the subject line, the subject line gets encoded as UTF-8.

When you open the message in Outlook, Outlook displays the entire message as UTF-8 encoded, even if the message body is encoded in a different format. This will result in strange characters displaying in the Subject and Message body unless your email client is set to view in UTF-8 encoding
To view the email properly, on the Outlook toolbar, click View > Encoding > More > Unicode (UTF-8).

The EncodeSubjectLine key prevents Symantec Mail Security for Microsoft Exchange from encoding the subject line tag.