We're told that Imail is open relay by default, but can be closed easily. To stop open relay, on the Imail SMTP Security panel, click Relay options:Relay for Addresses and enter your trusted ip addresses and/or subnets. Then, on the Imail SMTP Security panel, UNcheck "Disable SMTP AUTH reporting" and tell all your mail users to use SMTP AUTH in their mail client programs.
More information is in Chapter 8 of the IMail 6.0 Manual (PDF, 2099K.), ftp://ftp.ipswitch.com/ipswitch/manuals/imail6.pdf.