Mailspinner based on other UC's. Emu and Mailspinner provide spam lookups. Relay prevention only on non-sendmail packages. Address book- no automated PIM interaction for the Low cost choices. None offer users automated password changes.
All commercial products offer support a limited time for free.
Potential for help with admin. Outsourcing shares administrative responsibility with us. These differences have complex impacts on installation effort, risk and cost, differing levels of support required and available, and architecting implications.