sending hostname の条件は成立しそうもない。

正逆一致したアドレスから送ることとSPF条件を満たすこととは両立づらい。DNS/運用/SPF

SPFレコードのチェックに使用されるドメインはMAIL FROMコマンドで送られるドメイン

• エンベロープFromとReturn-Pathヘッダに現れる。

1. IP addresses

Your sending IP address must have a PTR record. PTR records verify that the sending hostname is associated with the sending IP address.

Every IP address must map to a hostname in the PTR record. The hostname specified in the PTR record must have a forward DNS that refers to the sending IP address.

Set up valid reverse DNS records of your sending server IP addresses that point to your domain. Check for a PTR record with the Google Admin Toolbox Dig tool.

Important: The sending IP address must match the IP address of the hostname specified in the Pointer (PTR) record.

1.1. Shared IP addresses

A shared IP address (shared IP) is an IP address used by more than one email sender. The activity of any senders using a shared IP address affects the reputation of all senders for that shared IP.

A negative reputation can impact your delivery rate.

If you use a shared IP for sending email:

• Make sure the shared IP address isn’t on any internet blocklist. Messages sent from IP addresses on a blocklist are more likely to be marked as spam. If you use an email service provider for your shared IP, use Postmaster Tools to monitor the reputation of the shared IP address.

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