The big difference is a car with an engine that randomly shuts down is a life-and-limb safety issue. An email client that corrupts the database is extremely unlikely to cause a loss of life, even if the consequences are costly.
That said, even if the bug is impossible to isolate, it sounds like the chain of events that leads to it is known. They probably should disable the feature until someone is motivated to fix or replace the code. I'm sure that would anger a lot of people, but someone angry about the loss of a feature is probably better than someone who is angry at the loss of data. Especially given that the feature seems to be something someone would use to archive their mail.
That said, even if the bug is impossible to isolate, it sounds like the chain of events that leads to it is known. They probably should disable the feature until someone is motivated to fix or replace the code. I'm sure that would anger a lot of people, but someone angry about the loss of a feature is probably better than someone who is angry at the loss of data. Especially given that the feature seems to be something someone would use to archive their mail.