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My friend, I also wondered this question. I had a sulpheed account and I needed to move the message from there to my new outlook account. I broke my whole head trying to find any information until I turned to my father for help. In a few hours he found this instruction https://www.outlooktransfer.com/how-to-convert-sylpheed-messages-to-outlook/ how to convert sylpheed messages to outlook. This article helped me a lot, I hope that you will find the necessary information there.
I hope there are some tools and services in the internet, so you can look for it yourself. I also can't find such convertor to my documents and I'm waiting for another response.