I realized after some testing that neither Postfix (my MTA), nor Dovecot listened to IPv6 by default. In both cases, enabling IPv6 was easy.

(This is where I found out that my algorithm for getting parts of an entry and showing it on the front page doesn’t work that well.. so therefore, this text has been added, as it works as a workaround).

Postfix

Set the inet_protocols parameter in /etc/postfix/main.cf:

inet_protocols=all

The default value is ipv4.

Dovecot

Set the listen parameter in /etc/dovecot/dovecot.conf:

listen=[::]

At least on Linux, this will make Dovecot listen to both IPv4 and IPv6. Setting the value of listen to *, will make it listen only to IPv4.

Apache

My apache server happily picked up the presence of an IPv6 interface after a restart. This is probably due to the fact that I have a the following in my Apache configuration:

listen=*