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In order to use Crossware Mail Signature for Domino v5, you must be able to run 64 bit Domino. 

As this is now just a 64 bit product (inline with HCL/IBM as of version 10, the server is only available in a 64 bit now) so Crossware has been only created in a 64 bit.

Anything lower than 64 bits, will need Crossware Mail Signature for Domino v4.
Related Products: CMS DOM v4, CMS DOM v5