4.4.5

Enhanced Status Code 4.4.5: Mail System Congestion

Temporary failure Medium severity Network RFC 3463
What it means

Enhanced Status Code 4.4.5 means “Mail System Congestion.” The receiving system is too busy to take the message right now. Gmail sends it as 421 4.4.5 server busy. It is about their capacity, not your reputation.

At a glance
Code4.4.5
Bounce typeSoft (temporary)
SeverityMedium
CategoryNetwork
What to doQueued and retried automatically
StandardRFC 3463
What it looks like in your mail logs 
421 4.4.5 Server busy, please try again later

What does 4.4.5 mean?

Enhanced status code 4.4.5 means mail system congestion. RFC 3463 defines it as the mail system being unable to deliver because it was congested. Google returns it as 421 4.4.5 with text saying the server is busy and asking you to retry later.

This is one of the few deferral codes that genuinely is about the receiver rather than about you. A congestion response is capacity management: the server is shedding load and will accept the same message happily when it has room. Well-behaved MTAs back off and retry, and the message delivers with no intervention.

The trap is over-reacting to it. Because it looks similar to reputation-driven throttling such as 4.7.28, operators sometimes respond by reducing volume, warming IPs, or auditing authentication, none of which addresses congestion. The way to tell them apart is the text and the spread: congestion appears across many senders at once and does not name any dimension of your sending, while reputation throttling names your IP, domain or netblock specifically. If you are opening a very large number of simultaneous connections, however, you can genuinely contribute to it, and reducing concurrency helps.

How 4.4.5 plays out

Your server attempts delivery
The recipient defers with a temporary 4.4.5 reply
Your server queues the message and retries on a back-off schedule
It delivers on a later attempt, or becomes a hard bounce if it keeps failing

Where 4.4.5 sits: soft vs hard bounce

Soft bounce (4xx) Hard bounce (5xx)
NatureTemporaryPermanent
SMTP class4xx5xx
What to doLet it retrySuppress the address
Recoverable?OftenNo
4.4.5 is✓ this code

Common causes of 4.4.5

  • The receiving mail system is under heavy load from all sources
  • A backlog at the receiver is being worked through
  • Your sender is opening more simultaneous connections than the receiver wants
  • A maintenance window or partial outage has reduced receiving capacity
  • A shared hosting platform is throttling at the platform level rather than per sender
  • A traffic spike, seasonal or otherwise, is affecting the destination

How to fix 4.4.5

  • Let your MTA back off and retry, which resolves most of these without intervention
  • Reduce simultaneous connections to the destination if you open many at once
  • Distinguish congestion from reputation throttling by checking whether the text names your IP or domain
  • Do not reduce volume or start an IP warmup in response to genuine congestion
  • Check the provider status page if the condition persists across hours
  • Confirm your retry schedule uses exponential back-off rather than a fixed short interval

Frequently asked questions

What does 421 4.4.5 mean?
It means the receiving mail system is congested and is asking you to try again later. It is a temporary condition about their capacity rather than a judgement about your mail. Your server will retry automatically and the message normally delivers without any action from you.
Is 4.4.5 a reputation problem?
No, and this is worth being clear about because the response differs completely. Congestion is capacity management applied to everyone. Reputation throttling, such as 4.7.28, names something specific about you: your IP address, your netblock, your DKIM domain. If the text names nothing about your sending, it is congestion and reducing volume achieves nothing.
Can I cause congestion myself?
You can contribute to it if you open a large number of simultaneous connections to one destination. Most receivers limit concurrency per sender, and exceeding it produces congestion responses that look identical to genuine load. Reducing your connection count is worth trying if you are a high-volume sender seeing this consistently.
How long should I keep retrying?
Use exponential back-off and a normal queue lifetime, typically 24 to 48 hours. Retrying aggressively at a fixed short interval makes congestion worse and can look like abuse. If the condition persists beyond a few hours across all your mail to that destination, check their status page.
Why do I see 4.4.5 only at certain times of day?
Because receiving systems have peaks, and large senders often schedule campaigns at the same round hours. Shifting your send window even slightly away from the top of the hour frequently removes the problem entirely, and spreading a large send over a longer period is better practice regardless.

Related bounce codes

4.4.2 Bad Connection 4.4.0 Other Network or Routing Status 421 Service Not Available - Try Again Later 4.7.28 Mail Flood Detected - Temporary Rate Limit 4.3.0 Other Mail System Status - Temporary

Sources & further reading

RFC 3463 · The standard that defines this reply code IANA registry · The authoritative list of enhanced status codes and their meanings Gmail SMTP errors and codes · Google Workspace reference for every code Gmail returns
Reviewed by Jennifer Jackson, Email Deliverability Analyst · June 2026 ← All bounce codes