Hi,
I had a really quick look at EMU Mail, and I would be concerned about:
1. For regular CGI, on _every_ request, the program has to load 600k+ of perl code, plus decrypt all of it. This would add a lot of overhead. If you are using FastCGI or mod_perl this would go away, however this isn't an option on most [correction that should be non-dedicated] sites.
2. The script didn't compile for me under use strict, so I would be wary about running it under mod_perl (their readme says it may have problems).
3. No SQL support (it appears).
4. All source code is encrypted (I couldn't determine whether the purchased version includes source). Gossamer Mail includes full source code in readable, and more important, editable format. Editing the source code does not violate your license.
Hope that helps,
Alex
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Gossamer Threads Inc.