When sending email via Outlook using the server IMAP/SMTP which is relayed then to Amazon SES, there’s an spam marking on mail-tester saying “DOS_OUTLOOK_TO_MX Delivered direct to MX with Outlook headers”

Google says the following: “Both internal flags are harmless by themselves but their combination leads to DOS_OUTLOOK_TO_MX with high spam score. You need to check X-Mailer header of messages sent with PHPMailer. This header should be set to PHPMailer, not to Microsoft Outlook or something similar.”

Can it be forced through the panel to mark all email as PHPMailer or how do I avoid this spam marking while using Outlook?

This seems to be an issue with Thunderbird and Outlook, other email clients like Spike do not have this problem.