Another email address could be for many reasons, and could have been deliberately created or has come with an account from something.
e.g. My primary email address that I use doesn’t actually exist. (Currently - as the provider is changing their legacy systems and I will get a free years subscription when they migrate to the new system)
It relates to a web domain address I own, and the email works as forwarding addresses
I have a selection of email addresses on that domain which forward to the ‘real’ address that I use. I have changed the real address provider a couple of times, and updated as I moved
Other addresses that I have or have had are my internet providers email format, some fairly randoms which I give to some places on the internet so they don’t associate with the real me or so they can be discarded if they end up with spammers etc.
Others that I have are a Microsoft account (I register with my primary but can receive email addressed to a microsoft address and an apple icloud address
Having just looked at my iCloud address there are ‘game’ emails on there, which might tie in
But I don’t have my Microsoft, iCloud or my random disposable addresses configured on my email apps - which means i don’t get notifications if something uses them.
For a second email address to pop up a notification then it would need to be configured on the phone etc