[pit234a]

Hi nibbles.
Here is one of those, who visited your post and did not reply.
My experience with busybox is almost all leaned by the usage of my sat-receiver. This is a so called dreambox and there are plenty of add ons available. I don't remember mail servers however, just clients.
The problem with busybox is, that it is no fully featured OS, no GNU. It is different. All the commands are links toward the one binary busybox and this is compiled with all options, that it can deliver later and is not easy to change. Additionally, you can compile programs, but cross-compiling and there you would have to use the NAS System as reference. In the world of dreambox, where the software is widely developed in common Open Source manner, you find all you may need for this tasks, as CVS trees with the latest developments. I don't see this for Thecus and I would not know how to handle this. I never learned that stuff.
One cannot take any mailserver from elsewhere and build it in that NAS. There are a lot of libraries and even commands used in busybox, that do not offer all the options, that you may need and there are also a lot of libs not existent, that you normally find.
My idea therefore is since some time, building an own NAS or changing the Thecus to better serve me. Maybe, that could be worth considering for you also?
On a larger CF card, a fully featured GNU/Linux could easy be installed and used, instead of the small and restricted busybox.
I didn't have the time yet to investigate in this project. If the BIOS of the NAS is reachable, I could imagine even an alternative System booted from USB-stick. I don't also know about flash-memory. Is there some or is that just a part of the flash-card in the NAS, what is handled ro. Because I know, what I want get from my NAS, I know, that I could easy live with some basic installation of NFS and ftp-servers. For me, Thecus is working as expected and I don't miss anything, but I had a hard time to get it to that point and I would have been through it in a rush, when I would have been able to use a system, as I normally do, with ssh and easy configuring the files I want, instead using a webif where I cannot find things and when, they would not work as expected.