When requesting an SSL Certificate you have to prove that you own or you have management rights over the domain or sub-domain that you are requesting an SSL Certificate for.
Domain validation by email
If you have an SSL Certificate issued by Comodo, GeoTrust, Thawte, Symantec and RapidSSL, then you can complete the domain validation is by responding to an automated domain validation message sent to your email address. You will be given a list of emails to choose from, and the automated domain validation message will be sent to the email address that you choose.
Always check your email address (including your Spam folder) so as you should receive an email message from the Certificate Authority with instructions on how to validate (prove the ownership of) your domain name. The email message will ask you to copy a unique code and paste it on a specific link provided in the same email message.
If your SSL Certificate is issued by Comodo, there are 2 more ways how you can complete the domain validation:
Domain validation via the HTTP / HTTPS method
The HTTP validation consists of uploading a TXT validation file to a pre-defined location on your website. You have to make sure that you can access this file and link from any web browser. Once you proceed with this domain validation method, Comodo will run a scan of your website and will look particularly for this file at the given link. Your SSL Certificate will pass the domain validation within a few minutes after Comodo’s system finds the TXT file on your website.
The HTTPS validation method is the same validation method as described above. You should choose the HTTPS option if you already have an SSL Certificate installed on your website.
Domain validation via the DNS method
Comodo allows you to to add a pre-defined domain record to your domain registrar (the website where you registered your domain name). Make sure that your firewall doesn’t block Comodo validation robot. Comodo validation robot comes from secure.comodo.net / 126.96.36.199. The user agent should be “Comodo DCV” or “COMODO DCV”.
Please note that newly added DNS records take between 10-48 minutes to propagate. This means that you will have to wait up to 48 hours to pass the domain validation if you go with this method. That is why we recommend the Email, HTTP, and HTTPS methods better, so as they would allow you to pass the domain validation instantly.
How to change the domain validation method?
If you chose one of these domain validation methods described above, and you see that your domain doesn’t get validated, then you can always change your domain validation method. Please go to this link to learn how to do that.