Soft bounce

A soft bounce is a temporary email delivery failure in which the recipient's server accepts the attempt but rejects it for a transient reason. Unlike a hard bounce, the address remains valid and existing — the problem is momentary, so a later send attempt has a real chance of success.

What a soft bounce is

When the sender's server tries to deliver a message over the SMTP protocol, the recipient's server replies with a status code. Codes in the 4xx range indicate a temporary error and are exactly what produces a soft bounce. The most common causes are:

• a full inbox (storage limit exceeded),
• a briefly unavailable or overloaded mail server,
• an oversized message or attachment,
• a temporary hold by a spam filter (greylisting).

Mail-sending systems usually retry delivery of a soft-bounced message automatically for some time. If the address keeps bouncing after a series of attempts, it is treated as inactive and reclassified as a hard bounce so it does not damage the sender's reputation.

Soft bounce in practice

Distinguishing soft from hard bounces is crucial in email marketing and when analyzing deliverability. Mailing platforms report soft bounces separately so a marketer does not immediately delete valid contacts, only monitors those that bounce repeatedly. Keeping the bounce rate low and quickly retiring problematic addresses protects the sending domain's reputation and keeps campaign performance high.