MX レコード
Tells the world which mail servers receive email for a domain.
MX レコードとは?
An MX (Mail Exchanger) record points to one or more mail servers, each with a numeric preference (lower = higher priority). Sending mail servers query MX for your domain, then deliver SMTP traffic to the lowest-preference host.
ゾーンファイルの例
example.com. 300 IN MX 10 mx1.example.com. example.com. 300 IN MX 20 mx2.example.com.
代表的な用途
- ▸Outsource mail to a provider (Google Workspace, Microsoft 365, FastMail, ProtonMail).
- ▸Set up failover mail with a secondary mail exchanger.
- ▸Run an in-house mail server alongside a backup MX.
- ▸Validate "deliverability" — many anti-spam systems demand a valid MX before accepting mail.
よくある落とし穴
MX targets must be A/AAAA records, NEVER CNAMEs (RFC 2181). Avoid pointing MX at an IP — it must be a hostname.