You can use Notepad or MS Excel for Windows or Libre, WPS or other software for Linux. That helps you to organize all of your emails in one file. But don't upload it to the Internet because we don't know how safe their protection is. You can keep it in your computer or other places you think are safe so you will not worry about anything. Making it offline will be better.
Gmail alone works well for even though I have multiple emails from my casino account that I created long time ago.
Gmail has the feature of centralized inbox that let you see all the email of your account and merge it on single mail. Just clicking the drop down to check the receiver mail is what I’m doing to determine which is the correct email for the specific casino account.
This is much simpler than using other 3rd party apps that is not accessible anytime for me.