421 4.7.0: Try again later, closing connection

What 421 4.7.0 means

The message Gmail attaches to this code is “Try again later, closing connection.”, as written on its reference page. The first digit says how final it is: a 4xx reply is temporary, so the sending server queues the message and retries. The enhanced status 4.7.0 says what class of problem it is: rate limit. The sender exceeded a published cap or was throttled. Slow down and spread the volume; the cap resets.

Why it happens

The sender pushed more mail at the receiver, in a burst or across the day, than it is willing to take from a sender it does not know well. The limit is not only the published cap: a new domain sending its first hundred messages in one hour is throttled where an established one would not be.

What to do first: Connection-level throttling; retry with backoff.

Why warming does not fix this

Warming builds sender reputation. 421 4.7.0 in this form is a rate limit problem, and no amount of engagement history changes it. Fix the cause and the bounce stops; warming is for what comes after, when the mail is accepted and the question becomes which folder it lands in. See how warming works for that part.

How to fix it, in order

• Slow the send and spread it across the day
• Check the provider's published limit
• Start outreach at 20 to 30 a day per mailbox and scale with placement, not with the cap

Related codes

• 452 4.5.3 (Gmail): too many recipients
• 550 5.4.5 (Gmail): daily sending limit exceeded
• 550 5.7.1 (Gmail): rate limited
• 421 4.7.0 (Gmail): very low sending IP reputation
• 550 5.7.233 (Microsoft): tenant external recipient limit

More on Gmail

• Google Workspace emails going to spam: the Postmaster-led diagnosis, dashboard by dashboard
• Gmail and Yahoo bulk sender requirements: the rules behind Google's authentication and spam-rate rejections
• Google Postmaster Tools domain reputation: what the reputation tiers and the spam rate mean

Questions about 421 4.7.0