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In addition license under AGPLV3, hMAILSERVER can be used for free in most commercial scenarios. It’s source codes is available for anyone to read or extent at GitHub though.
In general hMAILSERVER comes with set of built-in anti-spam features such as SPF, SURBL. It can also be integrated with 3rd party anti-spam systems such as SpamAssassin, ASSP.
As a matter of fact any webmail system which supports IMAP and SMTP can be used with hMAILSERVER. Similarly RoundCube and SquirrelMail are popular web-mail systems.