Registration emails sent to certain addresses where "callout verification" is enabled will fail. The following delivery failure notice will be generated:
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
t231@recipient_domain.com
SMTP error from remote mail server after RCPT TO:<t231@recipient_domain.com>:
host recipient_domain.com [xxx.22.96.xxx]: 550-Verification failed for <wordpress@blogdomain.com>
550-Previous (cached) callout verification failure
550 Sender verify failed
This was not an issue with WP 2.2, but occurs on an upgrade to 2.3.1, or a fresh 2.3.1 install with no plugins enabled and the default theme.