I have indicated that I have been running my mail server on a Cubietruck.
It is running well enough for the past 270+ days since last boot. It supports 4 domains and receives ~5K messages per day, most spam. There are around a dozen users of this server. So not a big server, but what may be common for in a SOHO environment.
My one complaint is how the anti-virus scanning software can temporarily tie up both cores. This is sometimes noticable when I (or one of the other users) send an email (via imap - dovecot) and wait frm some seconds until the message starts to upload. Since getting mail is typically a background function, delays caused by big anti-virus scans don't show. Same with the actual receiving and sending of mail by postfix.
So I would recommend using a 4 core board, like the Wandboard, and somehow limiting the anti-virus software to 3 of the cores so one is available for actual mail work. I don't know how to limit services to specific cores so this would work. I suppose that even on the Cubietruck, you could limit the anti-virus to one of the cores. Perhaps Cubietech will actually come out with their 4 core server board as they have hinted at on their site, and we will quickly have a uboot to support it. Until then the Wandboard looks really good (and I think there is a BananaPi or OrangePi that is a 4 core with sata).
Of course it may be once I get Centos7 running with this software mix, I will be pleasantly supprised that things are better scheduled so that imap uploads do not get held up by other tasks like anit-virus scanning of big attachments.
Or someone here will show me a simple way to change service priorities to get things working better.
I think this is the only real performance issue I have on my server. But then I have not tested DANE yet either...