Man... now a days, it's impossible for a modern web app to not use JSON parsing. Can't see this as problem for a password manager. If you can't trust in the browser, you are dead. Ok, Kasp want to sell they fish, but, we need good arguments.
Take a look at AWS SES (https://aws.amazon.com/pt/ses/) you can send 100.000/month for only $3.8/month. And you can just configure it as relay-service for your smtp servers.