Your email campaign just finished sending, and you're looking at the results. Delivery rate: 94%. That means 6% bounced. But what does that actually mean? Are those addresses gone forever? Should you try again? Will continuing to send to them hurt your reputation?
The answer depends entirely on the type of bounce, and getting this wrong has real consequences. Remove addresses too aggressively, and you lose valid subscribers. Keep bad addresses too long, and you damage your sender reputation. The distinction between hard and soft bounces is the key to getting it right.
Hard bounces: The permanent failures
A hard bounce means the email address is permanently undeliverable. The most common cause is simple: the address doesn't exist. Maybe it was mistyped during signup. Maybe the person left the company and their account was deleted. Maybe the domain itself no longer exists.
When you get a hard bounce, the receiving server is telling you definitively: this address is not valid, don't try again. The SMTP error codes are typically in the 5xx range with specific subcodes like 5.1.1 (bad destination mailbox) or 5.1.2 (bad destination system).
The correct response to a hard bounce is immediate removal from your list. Not suppression for a while—permanent removal. Continuing to send to hard bounced addresses signals to email providers that you're not maintaining your list, which is a spam indicator. It also wastes resources and skews your metrics.
Some senders worry about removing addresses that might have bounced due to a temporary server misconfiguration. This is theoretically possible but rare. If you're concerned, you can wait for a second hard bounce before removing, but don't wait longer than that. The reputation cost of sending to invalid addresses outweighs the risk of losing a rare false positive.
Soft bounces: The temporary setbacks
Soft bounces are temporary delivery failures. The address exists, but something prevented delivery right now. Common causes include: mailbox full, server temporarily unavailable, message too large, or the receiving server is rate-limiting you.
The SMTP codes for soft bounces are typically in the 4xx range: 450 (mailbox unavailable), 451 (local error), 452 (insufficient storage). The server is saying: I can't accept this now, but try again later.
Your email platform should automatically retry soft bounces. Most systems retry several times over a period of hours or days, with increasing intervals between attempts. If the temporary problem resolves, the email eventually delivers.
The tricky question is what to do with addresses that soft bounce repeatedly. A mailbox that's been full for six months is effectively a hard bounce. A server that's been 'temporarily' unavailable for weeks probably has deeper problems. Most platforms convert repeated soft bounces to hard bounces after some threshold—typically 3-5 consecutive soft bounces over a period of days or weeks.
The gray areas
Not all bounces fit neatly into hard or soft categories. Some situations require judgment.
Policy rejections are increasingly common. The receiving server accepts the connection but rejects the message based on content, sender reputation, or other policies. These might return 5xx codes (suggesting permanent failure) but could actually be temporary—your next email might be accepted if your reputation improves or the content is different.
Spam filter rejections are similar. A 550 'message rejected' might mean the address is invalid, or it might mean the spam filter didn't like something about this particular email. Without more context, it's hard to know whether to treat it as hard or soft.
Greylisting is a spam-fighting technique where servers temporarily reject emails from unknown senders, expecting legitimate servers to retry. This looks like a soft bounce and should be handled as one—your server retries, and the email eventually delivers. But if your system doesn't retry properly, greylisted emails might appear as permanent failures.
The best approach is to look at patterns rather than individual bounces. If an address bounces once with an ambiguous error, give it another chance. If it bounces repeatedly with different errors, something is fundamentally wrong with that address.
Bounce handling best practices
Good bounce handling is mostly about having clear policies and applying them consistently.
Process bounces in real-time, not in batches. The sooner you stop sending to invalid addresses, the less damage to your reputation. Most email platforms handle this automatically, but verify that bounces are actually being processed.
Distinguish between bounce types in your reporting. Knowing that you had 2% bounces is less useful than knowing you had 1.5% hard bounces and 0.5% soft bounces. The former indicates list quality issues; the latter might just be normal temporary failures.
Investigate sudden spikes in bounces. If your bounce rate jumps from 2% to 10%, something changed. Maybe you imported a bad list. Maybe a major email provider is having issues. Maybe your authentication broke. Don't just accept high bounce rates as normal.
Keep records of why addresses were removed. If someone complains that they stopped receiving your emails, you want to be able to say 'your address hard bounced on this date with this error' rather than guessing.
Consider re-engagement campaigns for soft bounced addresses before removing them. An address that soft bounced due to a full mailbox might belong to someone who still wants your email—they just need to clean up their inbox. A targeted re-engagement email can recover some of these subscribers.
The reputation connection
Bounce rates directly affect your sender reputation, but the relationship is nuanced.
High hard bounce rates are a strong negative signal. They indicate you're sending to addresses that don't exist, which suggests you're not properly validating signups, you're using old or purchased lists, or you're not maintaining list hygiene. Email providers interpret this as spam-like behavior.
Soft bounce rates matter less for reputation because they're often outside your control. A recipient's full mailbox isn't your fault. But consistently high soft bounce rates to a particular domain might indicate that domain is blocking you, which is a reputation signal.
The threshold for 'too many bounces' varies by provider and isn't publicly documented. As a general guideline, keep hard bounce rates under 2% and ideally under 1%. If you're consistently above 2%, you have a list quality problem that needs addressing.
Remember that bounces are a lagging indicator. By the time you see high bounce rates, the damage to your reputation may already be done. Proactive list validation—checking addresses before you send—is more effective than reactive bounce handling.
Frequently asked questions
Should I remove soft bounces from my list?
Not immediately. Soft bounces are temporary and often resolve. Remove addresses only after multiple consecutive soft bounces over an extended period—typically 3-5 bounces over 2-4 weeks.
What's a normal bounce rate?
For a well-maintained list, hard bounce rates should be under 1%. Soft bounces vary more but typically 1-3%. If you're seeing higher rates, investigate your list sources and validation processes.
Can a hard bounced address become valid again?
Rarely. If someone creates a new account with the same address, or if a domain comes back online, a previously invalid address could become valid. But this is uncommon enough that you shouldn't keep hard bounced addresses hoping for this.
Why do I get bounces for addresses that worked before?
People leave companies, abandon email accounts, or let domains expire. Email addresses have a natural decay rate of 2-3% per year. Regular list cleaning accounts for this churn.