And there's a new CNET entry that says 40M but... it references the venturebeat link as its source not any actual Zimbra statement.

http://news.cnet.com/8301-13505_3-10190071-16.html

EDIT:

The author of that says: "I talked with the company. They just haven't released the formal PR yet"

I guess we will see later. Interested to know what the number actually means and how on Earth that amount of recent growth could be possible.