Helping people with computers... one answer at a time.
Welcome to another casualty in the war against spam. ISPs are starting to use various techniques to limit the use of their mail servers to only their clients.
Depending on what your ISP allows, you may be able to reconfigure. On the other hand, while it wasn't the last straw, it's one additional reason that my former ISP is my former ISP.
The techniques ISPs use fall into a few buckets:
There are probably other techniques in use as well.
I'm going to assume from the question that you can receive email on both accounts, so I'll limit myself to sending issues. With all those possibilities, and with all the possible ISP combinations, it's difficult to present The Answer. But I can at least go over a couple of things that I'd try.
The normal configuration might be to have each account send email using the SMTP server provided by that account's ISP - by that I mean your Verizon mail account will use Verizon's SMTP server, and the Comcast account would use the Comcast server. Instead, try configuring the problem account to use the other accounts SMTP server.
For many years sending email via SMTP required no authentication. Especially when you're connected via the ISP's provided lines such as dialup or broadband, that was enough to "authenticate" that you are authorized to use the ISP's servers. Many still operate that way by default but impose some simple restrictions such as requiring that the "from" address be something that the ISP controls. Quite often the ISP will lift restrictions as long as SMTP authorization has been successfully used. Try configuring the account that's not working to use authentication - either "same as receiving account", if that's an option, or with explicit account/password information.
And it's even worth combining those two approaches: try sending using the "other" accounts mail server, and try adding authentication to that configuration as well.
If efforts so far have been unsuccessful, it might be worth asking one of your ISPs if they even support what you are doing. Some might allow you to send using a different port - something other than the SMTP standard port 25 - or they might have alternate approaches. Or they might not help at all, and you'll know it's time to find another ISP.