Before a week ago, I was also searching for a method or solution to convert Lotus Notes to MBOX file format. But I was unable to find the solution. Then my colleague recommended me to try TransferMyMails Lotus Notes Converter software. It is very helpful and effective solution in converting NSF files from Lotus Notes to MBOX format. After that, I have used this program to save my NSF files in MBOX file. And trust me, it gives me an accurate result after completing the process.
For completing DataVare NSF to MBOX conversion effectively, you have to surely get some third party application manager. You have to surely go for Lotus Notes to MBOX Converter which is an external utility so needs some investment but with my personal experience I can say that it is worth the money.