Intro #
If you’ve been sending out email newsletters for a while, you will eventually encounter email bounces, which happen when a recipient’s mail server rejects your message. These bounces are generally split into two categories: hard bounces and soft bounces.
By reviewing the bounced email notification, you can check the RFC status codes to determine the bounce type. A hard bounce typically features a 5xx code, whereas a soft bounce displays a 4xx code. Below, we will break down what causes each type of bounce and how you can mitigate them.
Soft Bounces #
Soft bounces are emails that are temporarily undeliverable due to transient issues on the recipient’s end. Common causes include:
- Full Mailbox: The recipient’s mailbox has exceeded its storage quota and cannot accept new messages.
- Server Downtime: The recipient’s mail server is temporarily down or offline.
- Greylisting: The recipient’s server temporarily rejects the first delivery attempt to defend against spam. (Since mass spamming tools rarely retry, this filters out malicious senders).
- Message Too Large: The email size exceeds the maximum limit allowed by the recipient’s mailbox.
Remedy: For most soft bounces, your mail server will automatically retry delivery over a few days before giving up. In the case of greylisting, the second delivery attempt is typically successful. Resolving full mailboxes or down servers depends entirely on the recipient clearing their space or their IT team restoring service, meaning you generally do not need to take immediate action. However, if an email fails because it is too large, it likely will never go through unless you reduce the file size of your content or attachments.
Hard Bounces #
Hard bounces represent emails that are permanently undeliverable. You will typically encounter a hard bounce for the following reasons:
- Non-Existent Address: The recipient’s specific email address does not exist.
- Non-Existent Domain: The recipient’s email domain (the part after the @) is invalid or has expired.
- Syntax Error: The email address contains formatting mistakes (e.g., typos like
john@gmaill.comor missing symbols). - Blacklisted Sender: Your mail server’s domain or IP address has been blocked by the recipient’s network or a major spam database.
Remedy: To prevent hard bounces, always run your mailing list through MailboxValidator’s bulk validation before launching a campaign. This filters out invalid, misspelled, or stale addresses, effectively eliminating the first three causes.
If your emails are bouncing due to a blacklist issue, your messages will continue to be blocked until action is taken. You will need to check major blacklist registries for your mail server’s IP or domain and follow the specific operator’s delisting procedures to restore your sender reputation.
Final Thoughts #
Managing email bounces is a critical part of maintaining a healthy sender reputation and ensuring your newsletters actually reach the inbox. While soft bounces often resolve themselves, ignoring hard bounces can severely damage your deliverability rate and land you on global blacklists. By consistently cleaning your subscriber lists and monitoring your server health, you can minimize bounce rates, keep the mail servers happy, and maximize the ROI of your email marketing campaigns.
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