Hi,
Please refer to the following documentation to know more about the working mechanism of Spam checker.
Use AI to check your message-content spam score (Dynamics 365 Marketing) | Microsoft Docs
And here is the website about how the scores are generated:
Apache SpamAssassin: Welcome
Generally, the spam check high score is a result of low text content in addition to other conditions that can be modified to increase the email deliverability.
For example, the original email, which only contains links or QR code, will be scored as high risk.

And I add some text into it. The scores will be lower.

The details:
- The hostname has a long, hexadecimal sequence
- The hostname has a long, non-vowel sequence
are related to the hostname of the URLs.
Here is a similar issue which you can refer to:
(+) URI hostname has long non-vowel sequence - Dynamics 365 Marketing Forum Community Forum
In other words, if you want to reduce the risk of this type of spam, you need to replace the hostname of the URL in your emails with something that looks more organized, and this also means that you need to try not to use features such as Files within Marketing, which generates URLs with this type of risk.