Well, to me honest I'm just about to start setting up software raid for my fileserver at home, so I'm not very experienced in it yet. Especially I don't know how good (or bad) the performance will be.
I did some research and apparently Linux Software RAID supports RAID 0,1,5, and 6. At home I will use two RAID 1 coupled together with LVM at the top which will give me the flexibility to expand the partition once it gets full.
I don't think software Raid supports hot swap (changing a broken HD with a brand new one on the fly), but you should better doublecheck - it might not be dependent on the software raid implementation but on the controller drivers of the kernel. Do you really need hot swap? Your mail server will have to be online 24/7/365/forever?
One of the reasons I will be using Software RAID is because I won't be bound to a specific RAID implementation of some product which might disappear in the future. You should take this as well into consideration - if you use hardware RAID through a motherboard or a separate controller and this will stop working after a few years, you won't probably be able to access your RAID without the original controller.
I PERSONALLY would try to avoid using the RAID controllers which are integrated in the motherboards - I never heard about anybody saying that they have a good performance and chances that a motherboard breaks down are for sure bigger than a single RAID card, as the motherboard incorporates more components.
What do you think?