Gmail consistently sending mailbox.org emails to Spam despite SPF/DKIM/DMARC passing

Emails sent from my email are consistently being classified as spam by Gmail.

I tested delivery to two separate Gmail accounts and both messages were placed in Spam. A plain text message with no attachment and a separate message with an attachment were both affected. Delivery to Proton Mail succeeded normally.

Gmail reports SPF: PASS, DKIM: PASS and DMARC: PASS. The messages were delivered via mailbox.org outbound server

Gmail’s reason is: “This message is similar to messages that were identified as spam in the past.”

Has anyone else had this? This is a significant issue for me because I need reliable delivery to Gmail recipients for my work emails correspondence. If mailbox.org can't resolve the deliverability problem, I unfortunately need to move to another provider. I really want to stay with Mailbox.org as I love it so far but this is a crucial problem for me.