I have followed the instructions to enable a domain key for my mail but every time it just says " DomainKeys are not enabled, as the text record validation has failed. Please try again."

Please can you tell me what I am doing wrong? I have read the instructions again and again and read everyone elses posts but it just won't work. 

And that means that every email I send to a gmail account ends up in their spam folder. Please help. My domain name is with onlydomains.com

Celeste.