The issue normally occurs when there is a problem with permissions in the Mailmax folder under c:\program files\Mailmaxse. Mailmax uses the default IUSR_machinename as the anonymous user to browse the Mail section of the website. It also requires this user to have Full Control for the two folder and its subfolders given below.
1> The Mailmaxweb folder under c:\inetpub\wwwroot\Mailmaxweb
2> The c:\program files\MailmaxSE folder
If the anonymous user does not have full access to these folders then he will not be able to view his mails properly via web mail.