Verizon wouldn't have anything to do with an app-specific problem. Verizon is only a service provider. It's either a Yahoo problem (that's where I'd place my bet), or a Google problem, since they're the ones making the operating system. More than anything, it's probably just poor development on the part of the Yahoo team.
A better question to be asking is why are you still using Yahoo? You have a Google phone; you're going to get the best reliability when you couple that with Google services.
I have Gmail too, but having had my Yahoo mail account for many years, it's a matter of convenience to keep it going as so many of my contacts have that address.
I too am having the same issues with Yahoo mail. I have had Yahoo mail for many years and have given the address to many clients. It woud cost be a lot of time and headache to switch to google mail.