If you have been creating trial tenants over and over again like I have been with Office 365 and their E3 license, it is likely you may run into the same issue I have.
Microsoft does not really give you any hints in regards to troubleshooting your domain verifications.
I am currently going after the MCSA for O365, and the way I have been able to learn the ropes of O365 for the 70-346 and 347, has been purely from signing up for their E3 trial.
No need to spend any money at all, and you can get Sharepoint, Exchange, SfB, etc. Additionally, you get 25 user licenses to play around with.
“What if my trial ends?” you say?
The good news is that you can extend the trial an additional 30 days! (totaling 60 days.) I have a single domain name I used to work with O365 and a lab environment at home that I setup an Exchange hybrid in, manage Sharepoint Online, etc.
Worst case scenario: You use your total of 60 days, and MS asks for money. Just buy another 99 cent throw away domain from godaddy.com or 1and1.com and start another trial. Once MS trashes your original domain which expired, (30 days after your original 30 day, or extended 60 day trial ends) you can use that same domain name again as an O365 tenant and start again.
Rinse. Repeat. Profit.