The fact that Bridge and its client can become desynchronized sporadically for some users is a high priority issue we have been working on. Bridge is open source, and as a result relies upon open-source components, and the root cause is an architectural issue in a library that Bridge uses to implement IMAP. When there are network issues, this library returns errors to email clients.
Unfortunately, there are hundreds of email clients, and some email clients don’t handle errors properly, and this leads to desynchronization.
Our error tracking shows this does not happen often (1-2% of Bridge users) and the symptom is usually incorrect display of messages or read/unread status which is fixed with an inbox resynchronization. There are cases where a combination of a desynchronized mailbox and a specific series of user actions can lead to accidental email deletion, but this is far rarer than desynchronization. Our implementation tries as hard as possible to avoid this. If you find you are missing an email, our implementation works around the issue by placing it in a users’ All Mail folder.
As Bridge is open source, updates on this issue have always been publicly posted on GitHub. Addressing this issue at the source requires replacing the core IMAP library. Unfortunately, there are no FOSS IMAP libraries that are sufficiently well maintained. Therefore, the solution is to build our own IMAP library called Gluon, which we have been focusing on since this issue was reported to us. You can follow the progress of this open-source project here: https://github.com/ProtonMail/gluon
We are not refusing to fix the problem. The only possible solution is writing a new open-source IMAP library which we can maintain ourselves to ensure this class of errors cannot occur again. We have doubled the size of the team working on this this year so it is a priority for us.
We’re confident that this addresses the main sources of desynchronization and will be available in the beta version of Bridge by the end of the year.