It should listen only on IPv6 on all modern OSes.
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branch : HEAD
# A space separated list of IP or host addresses where to listen in for
# connections. "*" listens in all IPv4 interfaces. "[::]" listens in all IPv6
-# interfaces, but may also listen in all IPv4 interfaces depending on the
-# operating system. Use "*, [::]" for listening both IPv4 and IPv6.
+# interfaces. Use "*, [::]" for listening both IPv4 and IPv6.
#
# If you want to specify ports for each service, you will need to configure
# these settings inside the protocol imap/pop3 { ... } section, so you can