Thanks for the thoughts.
I'lll take a look at the 5216 ref. manuals... as noted above, the 5320 implements BOTH the older SMTP commands, as well as the newer CSMTPSxx commands - which I decided to go with for future-proof compatibility - but if it gets too hard, I'll fall back to the older commands for now.
There are a number of error possibilities - and only these two show up...
8=NETWORK ERROR
255=UNKNOWN ERROR
...Are the least helpful... !
There are specific error responses for server, recipient, authentication etc - but they all seem to be for 'later' (!)