Since Atmail 5.5 DKIM support is added to the mail-server version of the software.
DomainKeys Identified Mail (DKIM) lets an organization take responsibility for a message while it is in transit. Technically DKIM provides a method for validating a domain name identity that is associated with a message through cryptographic authentication. This can assist with marking your emails as "trusted" and guarantee a greater level of delivery and less false positives with spam-filters. Providers such as Gmail and other large ISP's validate DKIM headers, and we recommend admins adopt this practice to help with message integrity.
Should you wish for all outgoing messages sent from your server to be DKIM signed, follow the steps below.

1: Make sure you are running Atmail 5.5, otherwise upgrade your copy to the latest version. Verify your server has DKIM support compiled into Exim:

/usr/local/atmail//mailserver/bin/exim -dd 2>&1 | grep Experimental_DKIM

This should return:

Support for: crypteq iconv() OpenSSL Content_Scanning Experimental_DKIM

2: Create a new private/public pair key via the cmd-line:

openssl genrsa -out /usr/local/atmail/mailserver/dkim.key 1024
openssl rsa -in /usr/local/atmail/mailserver/dkim.key -out /usr/local/atmail/mailserver/dkim.public -pubout -outform PEM

3: View the contents of /usr/local/atmail/mailserver/dkim.public

-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDEYVlzEzrHH1ile+IYBZasjVTi
n+kacOvmpiJGhxNuGKhTmOCrvLh4Z+eQp1Dvj7kJNUU3EF5nEbFl7WTb/Z3uxxET
MImk47xX2eJdr/q98c+gJurZvlbpFuTT9JhXRmA8kkHZrARHUpsWZMsNt69ewgQK
XaAKH1MH5I4y0+JsqQIDAQAB
-----END PUBLIC KEY-----

4: Remove the --BEGIN and --END tags, and remove line breaks so the public key spans a single line. Add the following to your DNS server zone file:

mail._domainkey.yourdomain.com. IN TXT "v=DKIM1; g=*; k=rsa; p=MIGfMA0GCSqGSIb3DQE............KXaAKH1MH5I4y0+JsqQIDAQAB"

5: Open the Exim configure file, locate the transport "remote_smtp" and append the following.

remote_smtp:
driver = smtp
#
dkim_domain=yourdomain.com
dkim_selector=mail
dkim_private_key=/usr/local/atmail/mailserver/dkim.key
#

6: Restart your nameserver and the Atmail services ( /etc/init.d/atmailserver restart )

7: Via Webmail or an external mail-client, send a message via SMTP to an outside address. View the headers of the email and if successful, you will see the header lines:

Dkim-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;d=yourdomain.com; s=mail; h=MIME-Version:Message-ID:To:Reply-To:Content-Type:Date:Subject:From; bh=4WWVzoOsUWW0f4BYC2VHYfz2dQAB9PwjbTcHsvaaHrY=; b=GvkmrnJM1f2MhkRxZjTwKIPWTYmVUm//P2hqSw4eJ7izAo0GFunTddhlZ4UOWfBiObJj7+E8OGcVjyoMKj+4bNVhPqaMEi3Iidzexn8uqYbM+1vGCUf7b1tg10C+dzfdnsQIiGrkAYYlMvWGefhDlRhFq0OQfI1sDYN7pMMoqeQ=
You can also test the DKIM header is successful by emailing dkimtest@atmail.org , on success you will receive an autoreply, otherwise a returned mail "Bad DKIM header"

--

This will be natively supported in Atmail 5.6 due Dec 2008 - In the meantime these changes can be made to any existing Atmail 5.5 installation with DKIM support.

If you are running CentOS/Fedora, by default the NFS daemon will only spawn up to 8 processes. If you have a heavily loaded system, with multiple frontends accessing the storage via NFS the performance will be seriously effected after 8 concurrent connections to the NFS store.

It is highly recommended to increase the default RPCNFSDCOUNT setting when the NFS daemon starts.

Under CentOS/Fedora, create the file:

/etc/sysconfig/nfs

Increase the default setting:

RPCNFSDCOUNT=32

32 is a ballmark figure, increase to 64 or 128 for a highly loaded system.

Next, restart the NFS frontend:

/etc/init.d/nfs restart

Then sit back and watch the performance and load improve on all your frontend servers running Atmail!

You can test the write performance via NFS from the command:

ssh root@frontend.system

su atmailcd /usr/local/atmail/userstime dd if=/dev/zero of=/usr/local/atmail/users/testfile  bs=16k count=1000

This will return the performance and transfer rate between a frontend and the NFS server.

16384000 bytes (16 MB) copied, 5.20865 seconds, 3.1 MB/s

For anything below 1 MB/s the NFS server is too heavily loaded and performance will be effected. Check your switch, your using a gigabit ethernet card, and the local disks on the NFS server are optimized.