You might know Protonmail as the security conscious email provider. And that's generally true, but after being a loyal customer for almost 5 years now, I've also watched them grow to be more and more user hostile. They keep launching auxiliary services that seemingly no one has asked for, like office apps (Sheets / Docs competitors), password managers, video calling apps, and more!
All while their focus on the mail application seems to have waned. The search experience is as bad as ever, it still doesn't support SMTP connections outside of their middleware desktop app, and the only way to get your data out of Protonmail is as a giant dump of thousands of individual .eml files.
So I finally decided to jump ship and move to Fastmail. I was actually a customer there almost 8 years ago, but quickly moved to Protonmail and hadn't looked back until recently.
#Migration
In order to get the many years worth of largely worthless correspondence out of Protonmail I seemingly didn't have many options. They have an export tool, but it fails to run on macOS Tahoe without manually removing the quarantine flag. After I got it to run I was shocked to find that it simply dumped all of my email into 1 flat folder of .eml and .json metadata files. There is also a labels.json that is exactly what it sounds like, a JSON blob of your label names and colors. And that's all you get.
You can take these individual files and drag and drop them onto the Fastmail inbox web UI, but then they all land in your Inbox without much in the way of retained organization or further metadata.
So Claude and I made a quick Python script to take this giant flat directory of email files and generate mbox files out of them. One for each directory you had in Protonmail.
Fastmail can then take these mbox files and import their entire contents into the target directory in one swoop. See Settings → Migration → Import → upload MBOX files.
If you want an easier path to move out of Protonmail to whatever other provider, check out protonmail-to-mbox.