The "latest update" (of AquaMail) was January 19.
I started getting messages about Verizon auth failures about two or three days ago.
Zero (zilch, nada) messages about this issue from last update until then.
And then "internal server error" really speaks for itself.
So yes, it's not the app, it's Verizon.
---
Being a mail app developer, I see some fairly major mail service break something about once a month (between them all).
Right now, in addition to this one, there a fairly major issue with Yandex (a major Russian web portal / search engine / mail provider).