Like @zEitEr says it depends how your node/vps is setup/configured , that is why i have asked your to run the test program on your VPS (I have send you an PM how to do it)
Despite you didn't want other persons to help you , i will try to help you :)
1. I your server resources dedicated or simply oversold node ? (want to share your provider? )
2. 4 GB is not much these days, did you install an resource program like hetrixtools to monitor your resources ?
3. any...
You had an internal license that only may be used by Dedicated and VPS providers which you are not clearly.
Maybe its an good idea to follow the links in this thread provide by DA staff instead of arguing with people who wants you help, just my 2 cents
Yes its because of the p=none; thing, which make your DMARC useless :)
This worked for me for vacation messages setup by roundcube with an strict DMARC