Unauthenticated email from [DOMAIN] is not accepted due to 550-5.7.26 domain's DMARC policy.
Written by Simone
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.