I've been trying to get my email accounts working with Zimbra, and it's definitely a lot easier to manage than Evolution or Thunderbird.
In any case, all of my accounts are validated except for my yahoo.co.jp account. (go figure)
Are Japanese accounts simply not supported at this time? After searching a bit, it seems to be the case, and that Chinese, Korean, French, UK accounts are validating correctly.
I'm using the Linux version Build 1537. (On Eeebuntu 2.0 NBR based on Ubuntu 8.10 if it matters)