Microsoft Account With Custom Domain; Can I Change It?

So I have a Microsoft account email that was given to me by a website I was previously affiliated with. I have no idea how this works, but somehow they were able to give me an email with a custom domain instead of @hotmail.com, @outlook.com, etc. I would love to get some info on ho this works because I can no longer contact the website and I am completely in the dark here but I believe the account uses Hotmail email servers. On Outlook, it says my email is a send only address, what does that mean, and why is it not send/receive? I would like to change my email to a regular @outlook.com email address but I cannot rename my primary alias, I think this is because of this weird account type with the custom domain. If I could get some personal help (I don't want to reveal my emails addresses here) that would be fantastic, I have been trying to figure this out for about a year now.