We didn't find a server (A Record) behind your hostname mailwizz.

Checking your DNS, looks like you need A record for mailwizz at naszawyspa.com, you have an A record for the main domain not where Mailwizz is installed, at least it seems like you don't. Based on the report you want to add A record within DNS with these values:

Name: mailwizz.naszawyspa.com or just mailwizz

ALso, you do not have a DMARC record but this isn't for the issue your asking about,but just add a TXT record to your domain _dmarc.naszawyspa.com with the following value: v=DMARC1; p=none

This is another site I like using for DNS, check out your DNS report: http://leafdns.com/index.cgi?testid=6E817637
Thank you @Clark. That site is very nice.
But I am confuse. I had DNS record mailwizz.naszawyspa.com -> Ipv4: on my website provider.
My VPS is on DigitalOcean. I added my domain to DigitalOcean DNS and setup this record.
Its look like:

And I have the same error.
I can't use SMTP because of my provider (after 5 bad mails - blocked sending), and I want to send emails from VPS on DigitalOcean.
Can you help me, where I should put this record?
So this should be how your DNS is setup based on hosting on another server and VPS as sending server.

Under the mail domain of naszawyspa.com
CNAME > mailwizz

CNAME: naszawyspa.com
server1.mierzwin.com < rDNS points to this so you make have this already

A record > mailwizz
IP where mailwizz is located

MX record is perfect already with rDNS
DKIM Passed so this is perfect
SPF passed so this is perfect
Dmarc > add this to pass validation even more, but not your main issue.

Make sure your NS are set with mailwizz.naszawyspa.com because Mailwizz is offline at the moment.

Something like this I would need access to an temporary email address of naszawyspa.com and see DNS from both sides VPS and other, screenshots are fine. Another option I would suggest is simply try another sub domain for MailWizz, for instance we use portal just ZIP Mailwizz and upload it to another sub domain, if its on the same server you won't have to change anything, it will use the same SQL DB, honestly that shouldn't be the issue. Its always great to test at least 3 different sources, to see if they all produce the same results try ItsNotSpam

Feel free to shoot me a message and I'll pass along my Skype info and hopefully fix the issue once and for all. I'll have you setup another delivery server within Mailwizz, and we can see if our server produces the same error and we could narrow down from there, plus if it works you could always use our servers to send @ $1 per 1k, so you do have options, if it doesn't produce same results then its on VPS side, if it does then its on Web Hosting side.