PureMessage for Microsoft Exchange only recognizes ASCII characters in the 'Specified Senders' area of the Block list. Therefore, any domain names you enter using extended (Japanese) characters will not be blocked.
A future release of PureMessage will support RFC3490 (Internationalized domain names), which prescribes using special combinations of ASCII characters to represent non-ASCII domain names.
Sophos Product and version
PureMessage for Microsoft Exchange, version 3.0.1 and above
Microsoft Windows Server (Japanese)