> Is there a great demand for jmap amongst your customers?

Only from enthusiasts, the main problem is that there are very few email clients supporting JMAP. Big players such as Google and Microsoft do not seem interested in JMAP so adoption has been painfully slow so far.

> What is stalwarts relationship to Fastmail, do they use stalwarts software?

No relation. Fastmail uses Courier IMAP as far as I know.