set DKIM signature algorithm to rsa-sha256 
https://www.rfc-editor.org/rfc/rfc6376#section-3.3

Index: scripts/dkimproxy_out.conf.example
--- scripts/dkimproxy_out.conf.example.orig
+++ scripts/dkimproxy_out.conf.example
@@ -8,7 +8,8 @@ relay     127.0.0.1:10028
 domain    example.org
 
 # specify what signatures to add
-signature dkim(c=relaxed)
+# for DKIM rsa-sha256 is mandated by RFC6376
+signature dkim(a=rsa-sha256,c=relaxed)
 signature domainkeys(c=nofws)
 
 # specify location of the private key
