Their taking this action in hopes of stemming the flow of SPAM from infected home personal computers.
I’ve never heard of Mail Submission (TCP Port 587) before this announcement so I had to Google for it and do a little research. It looks to me like Mail Submission is just SMTP with authentication although I could be wrong, perhaps someone would help clarify the benefits of using TCP port 587 over 25.
In any event, if you are a Verizon FiOS subscriber and you use a regular email client you’ll probably need to make some configuration changes.
You can find the official Verizon announcement here.