upgrade postfix package from v3.6.4-r0 to v3.6.6-r0 (see below)
https://web.archive.org/web/20220423135730/https://www.postfix.org/announcements/postfix-3.6.5.html
https://web.archive.org/web/20220423135734/https://www.postfix.org/announcements/postfix-3.7.1.html
https://git.alpinelinux.org/aports/commit/main/postfix?h=3.15-stable&id=af7b2b3e8cee40a98d1495c8f85defe449292385