Mandrill has enforced upon us that the only way to verify a sending-domain (domain used to send emails from) is by getting an email sent by them on an email of the sending-domain in use. So, if you use example.org or something.example.org to send emails, you can now only verify your sending-domain, by receiving an email from them on email@example.com or firstname.lastname@example.org in which you have to click on a verification link to validate your ownership of the domain.
Now that's not ideal. Add a TXT record is enough for verification purpose, and that's essentially the step that proves the ownership of domain through this step only but this is how it is.
The only issue with this approach is that one must have email hosting or an email inbox setup to receive emails on that domain. If you already have a working email on the sending-domain, you don't need to read the rest of the post. Simply, enter the working email for verification and get it over with. But if you don't, then instead of setting up email hosting somewhere for just verification purpose, you can utilize this trick I discovered. Its quick enough & free alternative to do so by simply using Inbound emails feature in Mandrill accounts.
Here is how to do it:
Add your domain under "Inbound domains"
Now add MX record for your sending domain as per the instructions
Add those MX records in your DNS and then validate those records are reflecting by clicking on "Test DNS settings".
Once confirmed, simply setup a route for that domain, by clicking the arrow on the right of "Test DNS settings" link.
Configure webhook POST URL to a URL you obtained on https://requestb.in
Now go to verification of send domains, enter the email address you configured and wait for the email to come at your https://requestb.in/XXXXXXX URL.
Find the link, copy and paste it in the browser.
And you are done!