The Yahoo Mail Inbox and the Yahoo Mail Address, are not the same thing :)

Well, I think that a Yahoo user who sign up for an Yahoo account should be able to link an external mail address without signing up for a Yahoo Mail Address and still be able to use the Yahoo Mail Inbox.

Some of the external mail address can be accessed throw POP or IMAP.
In this moment Yahoo Mail Inbox can retrieve the email from another account throw the POP protocol, but the user of the standard Yahoo Mail cannot send an email from the Yahoo Mail Inbox using the external email address.

I think that for this special case when a new user of Yahoo wants to link the new Yahoo account to an external mail address without signing up for an Yahoo Mail Address, this user should sent emails from the Yahoo Mail Inbox using the external email address.
(Sure the external email account should be activated, an email must be sent from mail-robot@yahooinc.com or something, to the external email address chosen to be linked with the Yahoo account).

Sorry if my comment is not truly related to your subject, but I have an idea an I must tell you (all) this idea.

In the sign up process all the web based email services from around the world ask you to take an ID.
This ID will be every time a part of your email address.

Let’s study the case of an Yahoo ID (the next sentences can be applied also for Google or AOL/AIM however)

For example, let’s say that I have the Yahoo ID: Mirela.
Knowing my (primary) Yahoo ID and my language preferences, it is very easy to guess my Yahoo Mail Address.
My mail address can be:
mirela@yahoo.com
mirela@yahoo.co.uk
mirela@yahoo.ca
mirela@yahoo.com.au
Sure, Yahoo use several domains, but still the email address is easy to be guessed.
On the other hand, I choose to have one address using this Yahoo ID (Let’s say mirela@yahoo.com), the other email addresses using this name before the @ remains unused.

So,
I sign up for a new account on Yahoo and I need a new mail address.
I took a new Yahoo ID, let’s say mirelanewyahooid2008, why cannot link this new ID to a mail address still available, still unused, like mirela@yahoo.ca?
Another question?
Why I cannot sign up for an ID with less than 4 characters after the dot? (Rocket mail something?)
If the Yahoo ID will be a different field in my profile than the Yahoo address, then this restriction will not be necessary anymore.

Thanks for your answer,
Mirela