Deliver emails ignoring domains
When you login to yahoo, it only checks the email id and not the domain. So that means one email id is unique across all domains. Assuming this is true, suppose one person has email id example@yahoo.co.XX, but if someone by mistake sends the email to example@yahoo.com, should it not still get delivered to the person as long as the mistaken domain exists? This does not happen today and the email delivery fails.
This is specially applicable as a lot of people here have similar (example@yahoo.co.XX) email ids but many times it happens that someone send it to example@yahoo.com instead of example@yahoo.co.XX.
In case this can't be done, then you should allow creating aliases or make example@yahoo.com id an alias for/linked to each yahoo.co.XX email ids created by default.

You’re correct in that all Yahoo IDs are unique. being as such, no two IDs can exist with the same username as different domains. So, if an email is sent to an address at .com, but in actuality it’s .co.uk, then it will bounce back to the sender as undeliverable.
Thank you!
Mail Support