Sun Java System Messaging Server 6 2005Q4 Administration Guide

ProcedureTo Set the Login Separator for POP Clients

Some mail clients will not accept @ as the login separator (that is, the @ in an address like uid@domain). Examples of these clients are Netscape Messenger 4.76, Netscape Messenger 6.0, and Microsoft Outlook Express on Windows 2000. The workaround is as follows:

Steps
  1. Make + a valid separator with the following command:

    configutil -o service.loginseparator -v "@+"

  2. Inform POP client users that they should login with + as the login separator, not @.