Here's the email I got from ProtonMail on the matter:

Dear ProtonMail user,

Starting at around 4:30PM New York (10:30PM Zurich), Gmail suffered a global outage.

A catastrophic failure at Gmail is causing emails sent to Gmail to permanently fail and bounce back. The error message from Gmail is the following:

550-5.1.1 The email account that you tried to reach does not exist.

This is a global issue, and it impacts all email providers trying to send email to Gmail, not just ProtonMail.

Because Gmail is sending a permanent failure, our mail servers will not automatically retry sending these messages (this is standard practice at all email services for handling permanent failures).

We are closely monitoring the situation. At this time, little can be done until Google fixes the problem. We recommend attempting to resend the messages to Gmail users when Google has fixed the problem. You can find the latest status from Google's status page:

https://www.google.com/appsstatus#hl=en&v=issue&sid=1&iid=a8b67908fadee664c68c240ff9f529ab

Best Regards, The ProtonMail Team

P.S. You might also consider asking your contacts who are still using Gmail to switch to ProtonMail for more private communications

Google has been having some issues lately, to say the least...