Hi Amit,
From the description, to address the issue, I want to collect some information:
1. Do you have two Office 365 service, Office 365 A2 plan and Office 365 A3 trial service?
2. Do you want to verify the custom domain to Office 365 A2 plan or A3 trial service?
3. Do you get the kind of error message "couldn't verify my domain…" when trying verifying domain to Office 365 service? If yes, would you minding providing a whole error message for me?
After checking the DNS records information of custom domain, I found that the custom domain seems to have been used for an Office 365 tenant domain service. About the DNS records information, I have sent a private message to you.
To see the private message, please follow the steps below.
1. Go to "Your details" section on the right side of the community site.
2. Click Private messages.
3. Click the subject title of the response to read the message.
In this situation, I learnt that you have used the custom domain for another Office 365 tenant service.
First of all, I’d explain that a custom domain can only be used for an Office 365 tenant service. When verifying a custom domain which has been used for an Office 365 tenant service to another Office 365 tenant service, we will get this kind of warning message "We can't verify the domain because it is associated with another Microsoft hosted service…"
Thus, if you want to use the custom domain to the current Office 365 service, you need to remove it from previous Office 365 service which the domain has been verified to, and then verify it to current Office 365 service.
Here is an related KB article for your reference.
"Can't verify domain" error message when you try to verify a domain in Office 365
http://support.microsoft.com/kb/2812454
For anything unclear, feel free to let me know.
Thanks,
Ida Qiu