diff --git a/cmdeploy/src/cmdeploy/postfix/main.cf.j2 b/cmdeploy/src/cmdeploy/postfix/main.cf.j2
index 1390456c..9a9ca20b 100644
--- a/cmdeploy/src/cmdeploy/postfix/main.cf.j2
+++ b/cmdeploy/src/cmdeploy/postfix/main.cf.j2
@@ -88,6 +88,22 @@ inet_protocols = ipv4
 inet_protocols = all
 {% endif %}
 
+# Postfix does not try IPv4 and IPv6 connections
+# concurrently as of version 3.7.11.
+#
+# When relay has both A (IPv4) and AAAA (IPv6) records,
+# but broken IPv6 connectivity,
+# every second message is delayed by the connection timeout
+# <https://www.postfix.org/postconf.5.html#smtp_connect_timeout>
+# which defaults to 30 seconds. Reducing timeouts is not a solution
+# as this will result in a failure to connect to slow servers.
+#
+# As a workaround we always prefer IPv4 when it is available.
+# 
+# The setting is documented at
+# <https://www.postfix.org/postconf.5.html#smtp_address_preference>
+smtp_address_preference=ipv4
+
 virtual_transport = lmtp:unix:private/dovecot-lmtp
 virtual_mailbox_domains = {{ config.mail_domain }}
 lmtp_header_checks = regexp:/etc/postfix/lmtp_header_cleanup