If the sender is using Exchange Online, this might explain why OOO replies would be marked as spam more frequently: https://github.com/MicrosoftDocs/OfficeDocs

If the sender is using Exchange Online, this might explain why OOO replies would be marked as spam more frequently: https://github.com/MicrosoftDocs/OfficeDocs-Support/blob/public/Exchange/ExchangeOnline/email-delivery/understand-troubleshoot-oof-replies.md https://github.com/MicrosoftDocs/OfficeDocs-Support/blob/pub... "By design, Exchange Online Protection uses the high risk delivery pool (HRDP) to send OOF replies, because OOF replies are lower-priority messages."