Pretty much all the email notifications I received from Veeam this year regarding general information such as service agreement updates are blank, such as:
If I select all text, then I can see the text:
It's as if the person working at Veeam is using a dark them with white text on black background but doesn't realize that the message is getting delivered with white text on white background.
I've got a few different Dev tenants that all received this email and it rendered fine on each of them. I suspect that as you have a processing rule setup to modify the email with a banner (as evidenced in your screenshot) that it is modifying the rendering framework of the email, hence the output you're seeing.
The email was actually sent as black text on a white background btw. As your email modification rule for the 'external email' rule uses white text on a coloured background, I believe that this is the root cause of your issue.
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Michael Paul
Veeam Data Cloud Solution Engineer - M365 & Entra ID