Yes, enabling IPv6 properly is recommended long-term, but first force outbound emails over IPv4 to quickly resolve the Gmail rejection. This error occurs because your Mail-in-a-Box (MIAB) server is attempting to send emails via IPv6 (address 2600:3c00::2000:98ff:fefa:77c), which lacks a valid PTR (reverse DNS) record matching its forward DNS, violating Gmail’s IPv6 guidelines.
It was not my intention to suggest this. But it does prove ownership of the domain to Google, enabling you to make use of the postmaster tools. If there is enough mail volume coming from your domain to gmail, the postmaster tools give you information on mail deliverability for your domain.
1 Like