Why emails bounce, and what it costs

A bounce is an email that comes back to the sender because it could not be delivered. One bounce is nothing to worry about. What matters is how many of your messages bounce, and what mailbox providers conclude about you from that number.

This guide covers where bounces come from, why they affect every email you send rather than just the ones that bounced, and how to keep them rare.

Hard bounces: the address will never receive anything

A hard bounce is a permanent failure. Sending the same message again tomorrow will not change the outcome.

The usual causes:

• The mailbox does not exist, or no longer does. Someone left the company, an account was closed, or a letter is missing before the @.
• The domain does not exist or does not accept mail. The domain expired, it has no mail server, or it was mistyped: gmial.com instead of gmail.com.
• The address is malformed. A space, two @ signs, a character that is not allowed.

The rule is simple: an address that hard bounces comes off your list right away. Writing to it again gains you nothing and costs you reputation every time.

Soft bounces: a temporary refusal

A soft bounce is a temporary failure. The address exists, but the message could not be delivered this time.

• The mailbox is full.
• The receiving server is down or overloaded.
• The message is too large, often because of an attachment.
• The server asks you to try again later, a common defense against bulk mail.

Most sending tools retry on their own. But a soft bounce that keeps coming back campaign after campaign ends up meaning the same as a hard one: a mailbox that has been full for months is usually abandoned.

Some refusals have nothing to do with the address. Your domain or sending IP may be blocked, or the message may look like spam. Those count as bounces too, but no address verification will fix them: the place to look is your sending setup, from domain authentication to reputation and content.

What bounces cost a sending domain

Your sender reputation

Mailbox providers and anti-spam organizations watch how your mail is received. A high share of nonexistent addresses is the signature of an old, bought, or carelessly collected list. According to the Amazon SES documentation (opens in a new tab), a high bounce rate can send your email to the spam folder instead of the inbox.

This reputation belongs to your domain and your sending IPs, not to one campaign. A single campaign to a dirty list can push your next emails toward spam, including the ones going to perfectly real customers. And if your transactional email (receipts, password resets) goes out from the same domain, it takes the hit as well.

Your account with your sending platform

Sending platforms watch each account's bounce rate, because one careless sender hurts the reputation everyone else shares. Amazon SES publishes its thresholds:

Your budget

Many email marketing tools charge by the number of contacts or by sending volume. A dead mailbox then costs you exactly as much as an active customer, every single campaign.

Signups are worse: a mistyped address in a form is an account that never gets activated. You paid to acquire that person and wrote the welcome email, and nobody will ever read it.

How to keep your bounce rate low

1. Verify at the door. When someone signs up, check the address and offer to fix an obvious typo before the account is created.
2. Confirm signups by email. Double opt-in, a link to click before anything else is sent, proves the mailbox exists and was typed correctly.
3. Never buy, rent, or share a list. Only email people who asked to hear from you.
4. Remove hard bounces immediately, and soft bounces that keep repeating.
5. Verify an old list before using it. A list that has sat unused for months almost always contains dead addresses. If you have to email old addresses anyway, start with a small slice of the list.
6. Check your rate after every send, not only when your platform sends you a warning.

Tips 2, 3, and 4, and the advice to start an old list with a small slice, also appear on the list Amazon SES gives its own customers for reducing bounces.

What PricklyMails tells you before you send

PricklyMails verifies each address without sending an email: the syntax, the domain and its mail servers, then, where possible, the receiving server itself. Every address comes back with a plain-language reason and a recommendation: send, review, or remove.

Google, Microsoft, and Yahoo do not reliably answer when asked whether a mailbox exists. Their addresses come back valid with a capped score and the reason shown, rather than with a confirmation those providers never gave. They are billed in full, mailbox test included: what you pay for is the answer that the address sits at a working provider. And when a server does not answer, the verdict is "not verifiable", never valid by default, and the tests that got no answer are refunded.

Have a list going out soon? Start with a few addresses: create an account, and the free tokens you get on signup are enough to try it, no credit card required.