Comment # 1 on bug 1186005 from Jason Record

I found two ways to resolve the issue as a workaround:

1. Update /etc/postfix/main.cf to change 

inet_interfaces = localhost

to 

inet_interfaces = 127.0.0.1

-or-

2. Edit /etc/hosts and delete the line:

::1             localhost ipv6-localhost ipv6-loopback

I found this issue on SLES15 SP1, but it is resolved in SLES15 SP2 and SP3.