ThumbOne - afraid I’ve given up on the Raspberry Pi as a mail server 
I’ve switched to an Intel NUC and (touch wood) it’s been fine, so far. (Still happy with MIAB.)
The pi seemed ok initially. I’m pretty confident the hardware was ok (I’ve since exercised it and the storage pretty well, without seeing any problems, under raspbian). But the server would crash about weekly! It might have been something to do with munin or one of the services, running wild - it would normally just die around 6am and need a hard reset.
My guess (without looking too far) is that it’s some subtle bug showing up in the pi ubuntu. Anyway, I changed to a NUC (still at home) and it’s been good…