Why can't I send an e-mail from my alias?

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A couple of months ago, I tried to create a new Yahoo e-mail address; I didn't intend to have it linked with my old e-mail address in any way, but it turns out that what I actually created was a Yahoo alias. Before I was able to figure this out, I set up an account at another website, giving my alias as my email address. I now need to send an e-mail to that website, and it needs to come from the e-mail address that I had given them.

I've determined that there are two types of aliases - alias IDs and alias e-mails. I'm not sure which one I created, but it's my understanding that if I have an alias e-mail, then I should be able to send e-mails from this address. I was told that there would be a "From" option while composing an email, but I haven't seen this anywhere. I've tried going to my e-mail inbox, selecting the gear icon, selecting "Settings", and
going to the “Default sending account” drop-down list, but only my original e-mail address is listed there.

If I sign into Yahoo with my alias, and then go to the gear icon and select “Account info”, it will take me to another sign-in page, where a small box displays my alias under the word “Username”. After signing in here, I am directed to a page with four boxes. The first of these boxes is divided into two sections – “Profile Information” and “Contact Information”. The “Contact Information” section displays my original e-mail address (highlighted in green), and next to this are the words, “Yahoo Email”. There is a green arrow on the other side of my address, and the word “Primary” (in green lettering) is next to the arrow. Next to these, is the word “Change” in blue lettering. When I click on the “Change” button, I am taken to a page with a heading that reads “Manage your email addresses”. The only e-mail address listed under this is my original one, but when I click the “Add Another” button, and enter my alias, I am shown a message that reads, “Looks like you're trying to add an email address that's already been added to your account. You're more than welcome to add another email address.”

So if this e-mail address has already been added to my account, then why doesn’t it display under the “Manage your email addresses” heading, along with my original e-mail address?

I’ve also tried going to my e-mail inbox, selecting the gear icon, selecting “Settings”, clicking the “Create an extra email address” button, typing in my alias, and then clicking the “Check Availability” button, but when I do this, I am told that it is unavailable. I tried sending an email to this address though, and received an error message, so clearly no one has actually registered an e-mail account with this address. If what I had created was a Yahoo ID, and no one else has an e-mail address by this name, then why would Yahoo be telling me that the address is unavailable?

Can anyone help me?