As reported on meta.trac it appears that smtp-fwd is lacking the rDNS which might explain why we’re seeing a higher level of slackSlack Slack is a Collaborative Group Chat Platform https://slack.com/. The WordPress community has its own Slack Channel at https://make.wordpress.org/chat/. invite failures of recent.
Could we add a reverse DNSDNS DNS is an acronym for Domain Name System - how you assign a human readable address to a website’s exact numeric coded location (ie. wordpress.org uses the actual IP address 184.108.40.206). for that mail forwarder please?
FYI; SPF and DKIM appear to be succeeding according to gmail.