OCI Email Delivery: Gmail rejects messages with 550 5.7.1 — shared IP reputation on Always Free

Summary

Gmail rejects legitimate personal emails sent through OCI Email Delivery with SMTP error 550 5.7.1 (UnsolicitedMessageError). I am using Oracle Cloud Always Free and would like to determine whether the issue is related to the reputation of Oracle's shared outbound IP addresses.

Environment

• OCI region: US West (San Jose)
• Account: Always Free
• Mail server: Mail-in-a-Box v76, Postfix
• Sending domain: bolt.linkpc.net
• SMTP endpoint: smtp.email.us-sanjose-1.oci.oraclecloud.com:587
• Outbound IP observed: 129.148.164.126
• Oracle DKIM signing: Active

Problem

On October 20, 2026, Oracle Email Delivery successfully accepted several legitimate personal test messages. However, Gmail rejected them with the following SMTP response:

550 5.7.1 — Gmail has detected that this message is unsolicited mail. To reduce the amount of spam sent to Gmail, message has been blocked.