You can ignore the warning quite safely, however, be sure to check that you have set up your SPF/DKIM and Domain Keys properly to ensure better delivery rates of your emails (to avoid them being marked as Spam).
Your current SPF for antagonist.nl is:
v=spf1 include:spf.antagonist.nl include:_spf.createsend.com include:_spf.google.com include:amazonses.com include:spf.patchman.co include:mail.registrar.eu ~all
In this case antagonist.nl is set up to use Google Mail servers (I can’t tell in what configuration, i.e. GsuiteApps account or forwarded to GSuite account).
I’m not entirely sure where the ‘submission’ part comes from as this would usually be the resolved domain name of the actual sending server.
Instead of your emails being sent from the IP address of mail.antagonist.nl, they are actually being sent by GoogleMail.com / Google.com, which will be a different IP address depending on your Geo Location. The reverse lookup on that GoogleMail/Google mail server in your case returned “Submission”
Hopefully that helps to explain it rather than confusing the heck out of you. Setting up Email correctly is a Dark Art !! I’ve been doing it for 30+ years and I’m still only at apprentice stage, every time I think I have handle on it, they go and change something !!
Some web hosting does some internal route and the mail server the client is getting is just an alias to the original one.
you can ignore it.
