postfix is using ipv6 on a server when ipv6 not available

Hi,
I have a postfix server managed by ISPConfig where sometime outgoing emails are deferred because postfix tries to connect to gmail.com smtp server using IPv6 address. The server has not ipv6 service.

I found a lot of (old) threads about disabling ipv6 in postfix, but I cannot find the actual best way to do this in ISPConfig and I'm confused, can you help me to fix this using the right/current way?

Any idea about postfix trying to connect to IPv6 addresses without this stack available on the host? This is the log:

2024-02-15T15:22:09.024619+01:00 myserver postfix/smtp[1450096]: D376422435: to=<[email protected]>, relay=none, delay=64, delays=4.1/0.01/60/0, dsn=4.4.1, status=deferred (connect to alt2.gmail-smtp-in.l.google.com[2404:6800:4003:c00::1b]:25: Network is unreachable)
2024-02-15T15:24:29.057103+01:00 myserver postfix/smtp[1450101]: 7D64A22434: to=<[email protected]>, relay=none, delay=735, delays=645/0.01/90/0, dsn=4.4.1, status=deferred (connect to alt2.gmail-smtp-in.l.google.com[74.125.200.27]:25: Connection timed out)
2024-02-15T15:29:28.683168+01:00 myserver postfix/smtp[1452189]: A1C4221C8C: to=<[email protected]>, relay=none, delay=630, delays=540/0.06/90/0, dsn=4.4.1, status=deferred (connect to alt2.gmail-smtp-in.l.google.com[74.125.200.26]:25: Connection timed out)
2024-02-15T15:29:28.684484+01:00 myserver postfix/smtp[1452182]: D376422435: to=<[email protected]>, relay=none, delay=504, delays=414/0.03/90/0, dsn=4.4.1, status=deferred (connect to alt2.gmail-smtp-in.l.google.com[74.125.200.26]:25: Connection timed out)
2024-02-15T15:38:59.079321+01:00 myserver postfix/smtp[1454246]: D376422435: to=<[email protected]>, relay=none, delay=1074, delays=1014/0.05/60/0, dsn=4.4.1, status=deferred (connect to alt2.gmail-smtp-in.l.google.com[2404:6800:4003:c00::1a]:25: Network is unreachable)
2024-02-15T15:39:29.111505+01:00 myserver postfix/smtp[1454248]: 7D64A22434: to=<[email protected]>, relay=none, delay=1635, delays=1544/0.05/90/0, dsn=4.4.1, status=deferred (connect to alt2.gmail-smtp-in.l.google.com[74.125.200.27]:25: Connection timed out)
2024-02-15T15:43:59.443699+01:00 myserver postfix/smtp[1455456]: A1C4221C8C: to=<[email protected]>, relay=none, delay=1501, delays=1440/0.06/60/0, dsn=4.4.1, status=deferred (connect to alt2.gmail-smtp-in.l.google.com[2404:6800:4003:c00::1a]:25: Network is unreachable)
2024-02-15T15:58:00.384931+01:00 myserver postfix/smtp[1458549]: D376422435: to=<[email protected]>, relay=gmail-smtp-in.l.google.com[142.251.31.26]:25, delay=2216, delays=2215/0.07/0.38/0.49, dsn=2.0.0, status=sent (250 2.0.0 OK 1708009080 n15-20020a170906688f00b00a3d72d77a6fsi696591ejr.401 - gsm

my ip config has only the link local ipv subnet:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host noprefixroute
valid_lft forever preferred_lft forever
2: ens192: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
link/ether 00:50:56:17:0e:03 brd ff:ff:ff:ff:ff:ff
altname enp11s0
inet 10.11.1.61/24 brd 10.11.1.255 scope global ens192
valid_lft forever preferred_lft forever
inet6 fe80::250:56ff:fe17:e03/64 scope link
valid_lft forever preferred_lft forever


thanks

 

thank you!