Como afirmado no erro, o valor do parâmetro smtpd_milters
e non_smtpd_milters
não estava correto. O caminho certo é
smtpd_milters = unix:/var/run/opendkim/opendkim.sock
non_smtpd_milters = unix:/var/run/opendkim/opendkim.sock
Como declarado em documentação oficial
The general syntax for listening sockets is as follows:
unix:pathname
Connect to the local UNIX-domain server that is bound to the specified pathname. If the smtpd(8) or cleanup(8) process runs chrooted, an absolute pathname is interpreted relative to the Postfix queue directory.
inet:host:port
Connect to the specified TCP port on the specified local or remote host. The host and port can be specified in numeric or symbolic form.