Just another suggestion, not all e-mail names are at the same name as the service. I.E., yahoo addresses can be "@yahoo.com", but also "@ymail", and "@fastmail", because big-name providers often buy out the smaller service providers, but keep the name.
Another thing, it could be your browser. I use Chrome, and since my e-mail is an "@ymail" address, whenever I sign up for a new website, I often get the prompt to correct it to "@gmail", and sometimes I click it without thinking.
Best of luck, I hope your issue gets solved.