Will my mail eventually get through?

Standard SMTP behavior is to retry a 4xx response, and most sending software does this automatically for a period (commonly up to several days) before giving up. But repeated deferrals mean something structural needs fixing, not just patience.

Is this the same issue as being blacklisted?

Not necessarily — a 421 deferral can happen to a domain with a clean reputation that simply sent too much too fast. Blacklisting is a separate, more severe reputation state; check with the blacklist checker to rule it out.

Gmail

Gmail 421 4.7.0 — temporary deferral, usually a reputation or rate issue

421 4.7.0

The 4xx class means "temporary failure — try again later," not "permanently rejected." Gmail is asking your sending server to back off, typically because of connection rate, sending volume that looks unusual for your domain/IP's history, or borderline sender reputation. Your server should retry automatically per standard SMTP behavior.

How to fix it

1
Slow down your sending rate
If you just increased volume or connection concurrency, reduce it. Gmail's rate limits scale with established sender reputation — new or suddenly-larger volume gets throttled.
2
Check your domain and IP reputation
Run the blacklist checker. A borderline-reputation IP triggers more deferrals under load even without being formally blacklisted.
3
Confirm authentication is clean
Deferrals often cluster with authentication problems. Check SPF and DKIM — an unauthenticated sender gets throttled harder than an authenticated one under the same conditions.
4
Warm up gradually
For a new domain or IP, ramp volume over 2–4 weeks rather than sending at full volume from day one.

Meaning current as of 2026-07-14, cross-checked against the provider's own documentation. Providers do occasionally redefine codes — if this doesn't match what you're seeing, the source link is the authoritative reference.

How to fix it

Common questions