I really think support people at Microsoft are using to much drugs. I had reported this problem. This is the answer I received today:
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We did investigate this issue in the back end and found that you can only have one personalized email domain per subscription. In this case as you have already set up deregt.in domain
you will not be able to remove or replace it with another domain. In order to manage two different domains you need to have two subscriptions.
Regarding the personalized email address option missing from premium settings, we are expecting a fix to be rolled out in next 24 hours. We will let you know once there is an update on the same.
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If I want to use another domain, I will have to subscribe on more time.
????
Regards,
Adrie de Regt