Check your dnsserver-<date and time>.log.
James records all errors related to resolving DNS for outbound mail to: /opt/collabnet/teamforge/james/james-<version>/apps/james/logs/dnsserver-.log. If you find that some of your TeamForge users are receiving email, but significant groups of others are not, you should consult this log to determine if James is experiencing difficulties in resolving their domain or MX records.