Bounce (Soft)

A soft bounce is when an email fails to deliver for a temporary reason, like a full inbox or a short outage at the recipient mail server. The address is still valid, and the email may be delivered if you try again later.

Soft bounce is when an email cannot be delivered for a temporary reason. The address itself is still valid. Common causes are a full inbox, message size limits, or a busy mail server. The email might get through on a later attempt.

Soft bounces matter because high bounce rates hurt your sender reputation. Inbox providers see a lot of bounces as a sign of poor list quality. That can push more of your messages into spam. Keeping soft bounces low helps protect your deliverability.

You can reduce soft bounces by cleaning inactive contacts and watching send volume. Many tools will retry soft bounces a few times before stopping. Use those reports to spot patterns, like one domain that always bounces. Pause or review those contacts instead of sending to them forever.

Where it shows up in bounces

SMTP 421Field manualService not available, try again laterRetrying
SMTP 450Field manualMailbox unavailable or busyRetrying
SMTP 452Field manualInsufficient system storageRetrying

Related terms

Bounce (Hard)Hard bounce is when an email cannot be delivered because the address is invalid or blocked. It is a permanent failure and the address should be removed from your list.
Email AliasAn email alias is a version of your email address that still delivers to the same inbox. It lets you create different addresses for signups and tracking without managing more accounts.
Email AppendingEmail appending is when you use a third party to add email addresses to your existing customer records based on other details like name or postal address. It is risky because these people did not ask to hear from you, which often leads to spam complaints and poor deliverability.
Disposable EmailA disposable email is a temporary address created for a single use, often to sign up for a service without revealing a real address.

More references.

SMTP field manual.

Every SMTP reply code, with real bounces from mailbox providers and spam filters and who needs to act.

Bounce playbooks.

Step-by-step fixes for bounces from Gmail, Yahoo, Outlook, Apple, and more, plus what each provider expects from senders.

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