At this time, Proton Mail is not compatible with SaneBox. Due to the way that this provider encrypts your emails, they are unable to support IMAP.
Proton Mail has assured their customers that they are working on a way to be able to use third parties and clients (hopefully including SaneBox) with their service. Learn More.
Yet, so far the IMAP “bridge” option offered by Proton Mail does not include any option for compatibility with web apps like SaneBox. It is intended as a bridge for your locally installed email clients only.
If you would like to speak with one of our support heroes regarding this, you may contact us.