Duelling standards and anti-spam measures / 2017-09-28

In today's mail problems:

   ----- Transcript of session follows -----
... while talking to ecp-nl.mail.protection.outlook.com.:
>>> DATA
<<< 450 4.7.26 Service does not accept messages sent over IPv6 [2001:980:14ca:61::13] unless they pass either SPF or DKIM validation (message not signed) [VE1EUR01FT036.eop-EUR01.prod.protection.outlook.com]
<info@ecp.nl>... Deferred: 450 4.7.26 Service does not accept messages sent over IPv6 [2001:980:14ca:61::13] unless they pass either SPF or DKIM validation (message not signed) [VE1EUR01FT036.eop-EUR01.prod.protection.outlook.com]
Warning: message still undelivered after 4 hours
Will keep trying until message is 5 days old

Rerouting mail for ecp.nl via xs4all servers in the hopes of getting it delivered.

Some research shows me that the xs4all outgoing mailservers (smtp.xs4all.nl) do offer incoming connectivity via IPv6 but don't connect to the IPv6 addresses of mailservers they are trying to reach.