I was enabling Exim on my IONOS VPS to deliver email through a smarthost and encountered the error message in this post's title (unable to send to Gmail users.. It's always them!)
After fiddling a while with DMARK and SPF I reconfigured Exim itself to rewrite sender address, so that emails coming from "spacenest.it" (IONOS domain) were sent as coming from "woodpeckersnest.eu", the smarthost and real email server.
Everything is done via this command:
# dpkg-reconfigure exim4-config
And the resulting configuration in /etc/exim4/update-exim4.conf is:
dc_eximconfig_configtype='smarthost'
dc_other_hostnames='cassandra.spacenest.it;spacenest.it'
dc_local_interfaces='127.0.0.1'
dc_readhost='woodpeckersnest.eu'
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost='pandora.woodpeckersnest.space::587'
CFILEMODE='644'
dc_use_split_config='false'
dc_hide_mailname='true'
dc_mailname_in_oh='true'
dc_localdelivery='mail_spool'
On OVH's postfix I just had to modify this line in /etc/postfix/main.cf:
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 94.143.138.27/32
where that last IP is the IP Address of my IONOS server.
I had previously discussed this issue on XMPP and on my Schleuder Mailing List, but there's news.
My ".space" domain is blacklisted by Google and others top email services because it is considered spammy - I still don't understand how you can blacklist a whole tld for spam but that's how it goes..
So I purchased a new ".eu" domain and set it up as virtual host in postfix.. But I was still using a third party relay to deliver emails to Gmail, Hotmail and iCloud. This relay works with "credits", each email you send using their free service is equal to 1 credit and you get 1000 credits per month (BIG Thanks to Kévin from Delta Chat for helping me out with this).
Now, I haven't ever reached the maximum allowed quota but I felt like removing the casters at least for Gmail, since this .eu domain should do the job just fine and because I cannot actually remove Hotmail, since they have my server IP banned for whatever reason.
So, I removed the gmail line in my transport file for postfix, ran postmap and reloaded postfix.service. Finally I sent a Ping over to "Delta Chat Italian Offtopic Group" where there are 2 people with gmail accounts and everything was fine, no email returned back to sender.
I'll keep going like this until problems!!
Now, if anyone is interested on how my transport file looks like, I'll share 😀
outlook.com smtp:[outbound.mailhop.org]:587
hotmail.com smtp:[outbound.mailhop.org]:587
outlook.fr smtp:[outbound.mailhop.org]:587
hotmail.it smtp:[outbound.mailhop.org]:587
mail.icloud.com smtp:[outbound.mailhop.org]:587
* :
You need an account on DuoCircle and you need to authenticate yourself (in postfix) for their smtp to accept emails from your server.