postfix won't restart

[steve%prd.co.uk@localhost (Steve Blinkhorn)]

This is NetBSD_9.2 amd64 on a server that is only used to send outgoing
automastic emails to clients when they have made use of our services.

Last changes to main/master.cf two years ago, otherwise running since
2016.  I can't tell when it fell over, but 'postfix start' fails with
no diagnostic error message.

Where to start?

-- 
Steve Blinkhorn <steve%prd.co.uk@localhost>