The mail provider belongs to the company 1&1 that operates the email provider GMX.com, Mail.com and much more.
Naturally, they have a strong protection against spam that requires for administrator to set up an trusted email sender, SPF rules and PTR record. Sometimes it might be an configuration which causes a bad email deliverability.
Cheers
Salmen
Each server I see forum emails coming from has a valid PTR and passes an SPF check... the SMTP banner probably does not match the PTR, which might effect some providers. I believe this would be something Amazon would need to change, depending on the type of account the forum has with Amazon, but I'm not really that familiar with Amazon's hosting, or what type of account the forum has, so I'm probably not going to be helpful.