There’s not currently a real time way to get that data, but it’s hardly “vendor lockin.”
You got there yourself, that's one of the problems.
There’s something ironic to me about chewing Proton out for alleged vendor lock in while using iOS / Apple products.
I used iOS as an example, for Android you can get a bridge but that's just going to be one more thing going for your battery.
Now, consider this, there's a TON of situation where having a standard SMTP-capable provider is interesting. Maybe you're running in iOS, maybe you want to have an ESP32 to send a few emails, or some custom software in your computer. All those use cases are impossible or require more coding and more non-standard solutions just because Proton decided to be the first provider ever not to use standard protocols.
What Proton is doing to e-mail is about the same that WhatsApp, Messenger and others did to messaging - instead of just using an open protocol like XMPP they opted for their closed thing in order to lock people into their apps. People in this community seem to be okay with this just because they sell the “privacy” cool-aid.