For DNS validation a CNAME entry has to be added in your domains DNS settings.
You can use the following tool to check our DNS CNAME entry:
http://mxtoolbox.com/SuperTool.aspx?action=cname%3Asrmxzuzop7zfmjb082ozccgd6t8hzhdx.my.domain.de&run=toolpage#
In this imaginary example the CNAME alias srmxzuzop7zfmjb082ozccgd6t8hzhdx.my.domain.de should be pointing to F3D9536B725A4303B664362A67965DB4BF836F71.comodoca.com
Example for testing your CNAME record in Linux:
nslookup -query=cname s90pb5wedhwjosstl6dyts6dvi1xix8u.ihredomain.de