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You may need to use Domainkeys for verifying emails and having them verified by Domainkeys-compatible mailservers (i.e., Yahoo.). Just do the following:

1.) Download the libdomainkeys library from: http://sourceforge.net/projects/domainkeys/

2.) Untar into atmail, then use make:

% tar xvfz libdomainkeys-0.69.tgz -C /usr/local/atmail/mailserver/; cd /usr/local/atmail/mailserver/libdomainkeys-0.69
% make

3.) If you have problems with dns_text, run:

% echo "-lresolv" > dns.lib; make

4.) Then, edit your Exim Makefile. This should be /usr/local/atmail/server_source/exim-4.69/Local/Makefile

5.) Add the following to the file:

EXPERIMENTAL_DOMAINKEYS=yes
CFLAGS  += -I/usr/local/atmail/mailserver/libdomainkeys-0.69
LDFLAGS += -ldomainkeys -L/usr/local/atmail/mailserver/libdomainkeys-0.69

6.) Find this, afterwards, in the file:
LOOKUP_LIBS=-L/usr/lib/mysql  -lmysqlclient

7.) Change to:

LOOKUP_LIBS=-L/usr/lib/mysql -L/usr/lib/ -lssl -lmysqlclient

8.) Then, go to /usr/local/atmail/server_source/exim-4.69, and rebuild Exim:

% cd /usr/local/atmail/server_source/exim-4.69
% make && make install

9.) Afterwards, open up your /usr/local/atmail/mailserver/configure file. Find:

remote_smtp:
driver = smtp

10.) Change this to:

remote_smtp:
driver = smtp
dk_selector = myselector
dk_private_key = /usr/local/atmail/mailserver/dk/rsa.private
dk_canon = nofws

11.) Afterwards, create your keys:

% cd /usr/local/atmail/mailserver
% mkdir dk; chown atmail dk
% openssl genrsa -out rsa.private 768
% openssl rsa -in rsa.private -out rsa.public -pubout -outform PEM

11.) Now, edit your named file for your domain in the server where your nameserver resides. For example: /var/named/yourdomain.

12.) Below the IN MX declaration, add:

_domainkey.yourdomain.com.       IN      TXT     "t=y; o=-"
myselector._domainkey.yourdomain.com.  IN      TXT     "k=rsa; t=y; p=[PUBLIC KEY]"

13.) Make sure you replace [PUBLIC KEY] with the contents of your /usr/local/atmail/mailserver/dk/rsa.public, without the end and beginning tags. For example, if your public key contains:

-----BEGIN PUBLIC KEY-----
MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhAL83KwACMaPsBvxqvdZM0UkSgZUDvjn8
r5xQAq9+Z46VGqH0fXbGLMAuvDX1ZMT2Pl62TUBvFcnGgmXVvbcr2iXBbWOhjGwZ
X5idkca3caB9+ZcT8dHJl4lnGTu8h4QWVQIDAQAB
-----END PUBLIC KEY-----

Your record should look like:

_domainkey.yourdomain.com.       IN      TXT     "t=y; o=-"
myselector._domainkey.yourdomain.com.  IN      TXT     "k=rsa; t=y; p=MHwwDQYJKoZIhvcNAQEBBQADawAwaAJhAKJ2lzDLZ8XlVambQfMXn3LRGKOD5o6lMIgulclWjZwP56LRqdg5ZX15bhc/GsvW8xW/R5Sh1NnkJNyL/cqY1a+GzzL47t7EXzVc+nRLWT1kwTvFNGIoAUsFUq+J6+OprwIDAQAB"

14.) Alter the serial, save the file, and restart named.

15.) Edit your named.conf file. In most installations, this is in /etc/named.conf. Add this to the options section:

check-names master ignore;

16.) So it should look like:

options {
directory "/var/named";
dump-file "/var/named/data/cache_dump.db";
statistics-file "/var/named/data/named_stats.txt";
         check-names master ignore;
};

This allows the "_" character in the records.

17.) Save changes. Restart @Mail on the @Mail server:

% /etc/init.d/atmailserver restart