I believe that DNS settings (at least for superuser.com) is incorrect.
TXT record for
SPF on superuser.com is (as of Jule 18, 2018):
superuser.com. 300 IN TXT "v=spf1 -all"
that tells all email servers in the world that domain
superuser.com - don't sent emails and asking receiving servers to reject any emails that comes as email@example.com, but in fact emails sent via
mx-out.stackexchange.com from user:
There also no
MX record for superuser.com that authorize email server that sends emails for domain superuser.com
So, as result receiving email server reject emails that sent kinda from superuser.com: (I replaced real recipient's email address for obvious reason)
policyd-spf: None; identity=helo; client-ip=22.214.171.124; helo=mx-out.stackexchange.com; firstname.lastname@example.org; email@example.com policyd-spf: Fail; identity=mailfrom; client-ip=126.96.36.199; helo=mx-out.stackexchange.com; firstname.lastname@example.org; email@example.com postfix/smtpd: NOQUEUE: reject: RCPT from mx-out.stackexchange.com[188.8.131.52]: 550 5.7.1 <firstname.lastname@example.org>: Recipient address rejected: Message rejected due to: SPF fail - not authorized. Please see http://www.openspf.org/Why?s=mfrom;email@example.com;ip=184.108.40.206;firstname.lastname@example.org; from=<email@example.com> to=<firstname.lastname@example.org> proto=ESMTP helo=<mx-out.stackexchange.com>
So, some1 @stackexchange who is responsible for email servers need to read this:
As a suggestion - all stackexchange's sites need to authorize mx-out.stackexchange.com as their primary MTA by adding to DNS MX record that point to mx-out.stackexchange.com and adjust SPF record that tells other servers in the world that mx-out.stackexchange.com or IP(s) of your MTA are only responsible to send emails for all forums.
Or at least set (I hope as temporary fix)
superuser.com. 300 IN TXT "v=spf1 ~all"
so emails would be tagged on receiving side as SPF rule violation but at least would be passed to recipient instead of telling other servers in the world to reject emails that comes from email@example.com.