Support for SMTP Networking is being phased out, so VPIM is the recommended way to go. Both SMTP and VPIM networking rely on SMTP, but SMTP Networking uses a proprietary message format, while VPIM is standards based. Also, VPIM provides some features that are not available with SMTP. Take a look at the following link.
http://www.cisco.com/en/US/products/sw/voicesw/ps2237/products_feature_guide_chapter09186a0080449a8a.html
Hope this helps. If so, please rate the post.
Brandon
