How about making it more secure! Won't it solve the problem? It's just not about Mailbox app it's about all the apps that should protect user's data. Should they care about their user's data or leave it up to the device to protect it?

No. Sorry but encryption doesn't really solve the problem. If you lose the device with valuable info on it, the info will be recovered even if it's encrypted.

Encryption absolutely solves the problem. Otherwise any kind of online security would be impossible.

You might need to use an actual strong password though and not the 4 digit passcode.