Kindly follow following Link to create webmail as subdomain:
https://help.directadmin.com/item.php?id=92
1) We'll need to add a webmail.* VirtualHost to each domain. We can make use of a feature that let's us globally specify CUSTOM# tokens, without altering the original templates.
Create the file
/usr/local/directadmin/data/templates/custom/virtual_host2.conf.CUSTOM.4.post
and add the code
</VirtualHost>
<VirtualHost |IP|:|PORT_80| |MULTI_IP|>
ServerName webmail.|DOMAIN|
ServerAdmin |ADMIN|
DocumentRoot /var/www/html/roundcube
CustomLog /var/log/httpd/domains/|DOMAIN|.bytes bytes
CustomLog /var/log/httpd/domains/|DOMAIN|.log combined
ErrorLog /var/log/httpd/domains/|DOMAIN|.error.log
<IfModule !mod_ruid2.c>
SuexecUserGroup webapps webapps
</IfModule>
taking note that we *start* the code with the closing </VirtualHost> because the CUSTOM4 token is before the trailing VH in the virtual_host2.conf template.
The above is for port http:80, but similar code can be used in virtual_host2_secure.conf.CUSTOM.4.post if you want it for https on the domain, you'd just need to add the lines after the opening <VH>:
SSLEngine on
SSLCertificateFile |CERT|
SSLCertificateKeyFile |KEY|
|CAROOT|
2) Then run
echo "action=rewrite&value=httpd" >> /usr/local/directadmin/data/task.queue
/usr/local/directadmin/dataskq d
to rewrite the httpd.conf files.
3) You'll also need to setup the dns portition.
cd /usr/local/directadmin/data/templates
cp dns_a.conf custom
cd custom
echo "webmail=|IP|" >> dns_a.conf
This will setup the webmail A record for new dns zones. For existing dns zones, you'll have to manually add the webmail A record to point to the domains IP.