Observed that Exim4 was listening on the IPv6 address. If you’re not accepting incoming IPv6 traffic, there’s little sense in having support enabled.

Per the Exim man page:

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$ man update-exim4.conf

# dc_local_interfaces
#      List of IP addresses the Exim daemon should listen on. If this is left empty,
#      Exim listens on all interfaces. Sets macro MAIN_LOCAL_INTERFACES only if
#      there is a non-empty value.

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$ grep dc_local_interfaces /etc/exim4/update-exim4.conf.conf
dc_local_interfaces='127.0.0.1 ; ::1'

What’s the ::1? Per the IPv6 Wikipedia article regarding Address representation:

The 128 bits of an IPv6 address are represented in 8 groups of 16 bits each. Each group is written as 4 hexadecimal digits and the groups are separated by colons (:). The loopback address, 0000:0000:0000:0000:0000:0000:0000:0001, may be abbreviated to ::1.

Therefore, remove the semicolon and the ::1 from the “dc_local_interfaces” declaration from the file /etc/exim4/update-exim4.conf.conf, to effectively disable IPv6.

The following is a differential comparison (‘diff’) to clarify the required change:

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# diff -U0 /etc/exim4/update-exim4.conf.conf.backup /etc/exim4/update-exim4.conf.conf
--- /etc/exim4/update-exim4.conf.conf.backup
+++ /etc/exim4/update-exim4.conf.conf
@@ -21 +21 @@
-dc_local_interfaces='127.0.0.1 ; ::1'
+dc_local_interfaces='127.0.0.1'

Restart Exim:

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$ service exim4 restart

Confirm only 127.0.0.1 (IPv4) is listening, by dumping socket information:

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# ss -ntlp | grep ":25"
LISTEN     0      20                127.0.0.1:25                       *:*      users:(("exim4",4998,3))