Interesting issue this week with Postfix and IPv6. The webserver was resolving MX records as usual for delivery of blog notification – and also looking up AAAA records for delivery via SMTP over IPv6. Without a functioning route from home to the far end these were obviously failing and not retrying with the (routable) IPv4.
The fix was to force Postfix to only use IPv4 – there’s a great readme on this.
/etc/postfix/main.cf: # You must stop/start Postfix after changing this parameter. inet_protocols = ipv4 (DEFAULT: enable IPv4 only)