3.2.1 update configured postfix on non-mail node
On a nameserver-only node, postfix is installed for local use, but was configured by the installer during a 3.1->3.2.1 update (so mail is now broken, with no amavis, etc.).
This was a debian 9 box. A second nameserver (mirror of the first one) has been updated to debian 10 and did not have this problem (postfix is installed there, too).