550 5.7.30 This message was blocked because it didn't pass DKIM authentication. Gmail requires bulk email senders to authenticate their email with DKIM.

Gmail blocked the message because DKIM did not pass. Bulk senders have to authenticate with DKIM.

421 4.7.30 is the rate-limited twin: DKIM authentication did not pass, and Gmail requires bulk senders to authenticate with DKIM.

The title is the 550 5.7.30 row on Google's SMTP error list, last updated 18 September 2026 UTC.