If you are experiencing problems with logging into SubAdmin due to session error try the following.

1> cat /etc/php.ini | grep session

2> Find session.save_path = "/var/lib/php/session"
This directory must be owned by the webserver user.

3> ps aux |grep httpd
To find the webserver username.

4> chown -R atmail /var/lib/php/session

Question:Our need is to block all email addresses/domain not configured in the @mail server.
So I want to avoid mail deliveries from addresses like bill.gates@microsoft.com or george.bush@whitehouse.org.

Answer:Yes this can be done. Follow the step detailed below.
Open and edit the Exim configuration file located at /usr/local/atmail/mailserver/configure, please remember to also back up this file before changes are made. Find and delete the following code.

# This router routes addresses that are not in local domains by doing a DNS
# lookup on the domain name.
dnslookup:
driver = dnslookup
domains = ! +local_domains
transport = remote_smtp
ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
no_more

Restart the @mail server and test.