Configs file for domein.com in '/opt/configs/[apache|nginx|lighttpd|hiawatha]/conf/master/domein.com.conf'.
If want modified, copy to '/opt/configs/[apache|nginx|lighttpd|hiawatha]/conf/master' and then modified this copy domein.com.conf with change 'ServerAlias mail.domein.com' to ''ServerAlias mail.domein.com mywebmail.domein.com' (example for apache).
After that, run 'sh /script/fixweb; sh /script/restart-web'.
Note: don't forget to add 'A record' or 'CNAME record' for 'mywebmail' in 'dns settings' for domein.com.