Subscribers to a thread (or moderators of a forum) receive an email notification for new posts. These notifications are successfully sent to the proper recipients.
The sender [i.e., "From:"] is:
"Forum at 188.8.131.52" <email@example.com>
But it should be from:
"Forum at www.go2japan.org" <firstname.lastname@example.org>
Essentially this isn't an error, since the string of numbers is the actual IP address of "www.go2japan.org".
Thus, I believe this will resolve itself soon, as the DNS registry is updated.
It's an inconvenience to the email recipient in that it's not clear the message is from go2japan.org. So it may be harder to file these messages until this problem works itself out.
EDITOR'S NOTE: This problem has been fully resolved.