Sieve extprograms ?not exexuting?

[E.B.]

> In another thread you said you are running CentOS. So I strongly guess
> it is SELinux interfering. Check your auditd log
>
> grep -i AVC /var/log/audit/audit.log
> 
> You can test whether your setup works after "setenforce 0". That sets
> SELinux into permissive mode, loggging AVCs but not blocking actions.

Good idea, but there are no AVC reports so I guess that's not it.
In the meantime I switched to calling the script using a dovecot
service and now the script isn't run at all ------ see my new thread
on that.