If you are using CloudFlare open DNS (1.1.1.1) services (like millions of other servers) then you may have noticed rapid increase of "open resolver" messages on your syslog indicating an email that did not go trough. It appears that spamhause.org blocks messages by default if it can not verify the sender DNS address quick enough. cat /var/log/syslog |grep "open resolver" Todays logs show lots of hits.
Yep. If you are using 1.1.1.1 for your DNS in NetworkManager (for example) you will probably be able to send email but your domains will no longer be receiving email (from outside you server). I discovered that today when revisiting a server I don't use too much. An error message I received from a Yahoo client when trying to send to an email address on my server was as follows: 554: 5.7.1 Service unavailable; Client host [106.10.242.140] blocked using zen.spamhaus.org; Error: open resolver; then a link to spamhaus which i had to delete in order to post this reply For me, the problem was solved by changing NetworkManager DNS to the googly 8.8.8.8 and 8.8.4.4 I imagine there are better long term ways/practices but for a simple one machine setup this worked for me.
