If you’re looking to verify if an email address is functioning properly there are a couple of methods available.
Content Error or Suggest an Edit
You can actually connect to mail servers using the telnet command. Here’s an article that sums up the process rather easily.
If you’re not familiar with using a terminal, which is now available on Windows then you can read this article that I’ve developed.
There are a number of online services that allow you to check if any email address is functioning.
If you know of others that work well, please comment and I will add them.