It has gone down again today.
Their statement yesterday:
Yesterday we unfortunately experienced technical difficulties which meant that Proton services were offline intermittently throughout the late afternoon and evening European time. During this period, incoming email, outgoing email, and push notifications were also temporarily delayed. As of 2:45AM Geneva time, all emails and notifications are caught up, but we are continuing to monitor the situation in case of further faults.
We can confirm that no data and no emails were lost. Due to our redundant infrastructure, emails that were not immediately delivered were queued and automatically redelivered. We are still investigating the issue, but what we can share so far is that the technical difficulties were caused by a software update that was conducted over the weekend which adversely impacted the performance of certain systems. Unfortunately, this issue only appeared under under high loads which we were not able to fully simulate in testing. Furthermore, because this was a major software and operating system upgrade, there was no way to roll back the software update.
This result of this was an extended period of intermittent outages and performance issues while we tried to work around the issue. We have now put a number of safeguards in place, and will continue to improve the reliability of our infrastructure to guard against this type of issue in the future. We apologize deeply for this issue and thank you again for your understanding. Our engineering team is currently conducting a root cause analysis and for transparency, we will be sharing results on the Proton blog after our investigation is complete.