I have encountered more than one site that has rejected my Gmail user name when, for example, creating a new account. The strangest one was yesterday, as I was changing an existing account from Centurylink to Gmail. As soon as I entered my Gmail userid in the box, it was rejected on the grounds that it was already taken.
I started a support ticket for two items, the first being the Gmail refusal, pointing out that, that the userid was in fact taken, as it was me. Given the large number of Gmail messages sent to my Gmail userid I get every day, it seems to me that that is a valid id, as well as the much smaller number of messages I send without their bouncing back as undeliverable.
The others usually just say “Error,” with which I agree, that they have made an error by not accepting my Gmail userid.
Ideas as to what is the cause, and perhaps also the cure, of this rather frustrating problem? It is particularly frustrating when I encounter it as I am in the process of changing an existing log-on from Centurylink’s email, which, when I have that mail sent to my computer to be used with Thunderbird email, a program vastly superior to Clink itself in terms of what one can do with the mail.