📩 The Real Problem Isn't Bounces

Ever have a subscriber go quiet and wonder if they vanished, ghosted, or got swallowed by Gmail?

Bounces are the trail of breadcrumbs. Read them right and you keep your spot in the inbox. Ignore them and your reputation slips.

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Hard vs soft bounces

Before you purge anyone with a failed delivery, pause. Some failures are permanent. Others are a request to try again.

Hard bounce = permanent failure
Think brick wall. This message will not land.

Why it happens:

• Address or domain does not exist

• Domain typo or malformed address

• Mailbox was deleted

• Blocked after a user complaint

What to do:

• Suppress immediately, no retries

• Log the reason and the code for future suppression logic

• Remove from any automations that would attempt another send

• Do not route through a different IP to “force” delivery

Soft bounce = temporary issue
The door is closed for now. It may open later.

Why it happens:

• Mailbox is full

• Receiving server is rate limiting or busy

• Message is too large

• Temporary spam filter or content check

What to do:

• Retry 2 to 3 times over several days

• If it keeps bouncing, suppress for a cooling period

• Review message size and sending cadence

• Watch engagement signals before you reintroduce

Bounce codes, decoded

Use the code, not your gut. A quick cheat sheet:

• 550 — Hard — mailbox not found or blocked

• 552 — Soft — mailbox full or over quota

• 554 — Hard — spam related rejection

• 421 — Soft — temporary system issue or deferral

• 4xx — Soft — generally retryable

• 5xx — Hard — generally permanent

What to do with bounced subscribers

Match action to evidence, not vibes.

• Hard bouncers — suppress right away, keep a reason tag for auditing

• Repeated soft bouncers — retry a few times, then suppress for a defined window

• Previously engaged users with temporary issues — handle with bounce-aware logic and a slower reentry plan

• Unknown patterns — quarantine to a diagnostic segment while you investigate

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Special case: Yahoo TS04 deferrals

Seen this one?

421 4.7.0 [TS04] Messages temporarily deferred due to user complaints

This is throttling, not a permanent failure. Common triggers include complaint spikes, sudden volume jumps, or shaky reputation.

How to handle:

• Throttle sends to Yahoo family domains (yahoo.com, aol.com, verizon.net)

• Send only to recently engaged users for that domain

• Retry gradually, not in one big burst

• Use Yahoo Postmaster data to monitor complaints

• Keep this segment separate until rates improve

Do not delete these subscribers. They are still valid. Treat it like a crowded doorway, not a dead end.

Special case: Gmail 552 “mailbox full”

You may see:

552 5.2.2 The email account that you tried to reach is over quota.

This is a soft bounce tied to storage limits that include Gmail, Drive, and Photos. The user is real. Their storage is stuffed.

How to handle:

• Retry slowly over 30 to 45 days

• If still bouncing after about 60 days, suppress and recheck monthly

• If this user was highly engaged before, give a little more runway before final suppression

Pro tip: suppress, do not delete. People clear space and come back.

ISP reference points

When in doubt, check the provider’s postmaster or error-code docs for the exact wording and policy. Look up Gmail, Yahoo, Microsoft, Comcast, and Spectrum. Policies move, the codes tell you how.

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• Smart Delivery — reputation tuning, domain alignment, and send-time strategy to keep you out of spam and in front of readers.

Want details? Book a call and we can map actions to your actual bounce logs.

Final thought
Every bounce tells a story. Sometimes it is “this address never existed.” Sometimes it is “you are pushing too hard at this domain.” Sometimes it is “their Gmail is stuffed with six years of cat photos.” Read the signals, act with discipline, and your emails keep showing up where it matters.

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