When you send an email to a recipient, it may be rejected by the recipient's server. In such cases, the email cannot be delivered—it “bounces.” This is referred to as Bounce.
The recipient’s server informs our system that delivery was not possible. The causes of a bounce can vary—there are no uniform standards, and each server may return its own error messages.
→ Note: In general, we recommend the measures described in the article Optimizing the Deliverability of Your CleverReach Mailings, to optimize the deliverability of your newsletters.
Bounces explained simply using the mailbox analogy:
Imagine your recipient list as a long street lined with numerous mailboxes, each representing the email addresses of your contacts. Every email you send is like a letter you want to drop into these mailboxes, and the mail carrier faces the following challenges:
| No bounce |
Successfully delivered The mail carrier was able to place the letter in the correct mailbox without any difficulty. The mailbox is present, accessible, and the name is correct. |
| Soft Bounce |
Currently Full The mailbox is full to the brim and has no room for additional mail. Once the recipient has emptied it, a redelivery may be successful. |
|
Temporarily Unavailable Access to the mailbox may be blocked due to construction work, or the recipient may be on vacation and the mailbox is not being emptied. | |
|
The recipient’s server is overloaded Imagine that the mailroom (the recipient’s email server) is currently under heavy load and temporarily cannot accept any new mail. |
| Hard Bounce |
Does not exist The address is simply unknown. The building was demolished or there was never a mailbox there |
|
Permanently Inaccessible The mailbox is secured with an unbreakable lock or completely bricked up. Delivery here is impossible. |
Below you will find an overview of typical bounce messages, which we frequently encounter at CleverReach:
→ For more detailed information on all possible error messages (including those specific to individual email service providers), we recommend visiting the website: https://smtpfieldmanual.com/
The SMTP Field Manual is a collection of raw SMTP error code responses from the major email service providers and spam filter services. Select your provider and look up the corresponding messages there.
Invalid address:
| Host is unknown / Host not found | The email server could not resolve the recipient's domain – it does not exist or is misspelled |
| invalid address / invalid recipient | The recipient's email address is invalid (e.g., incorrect format or typo) |
| Mailbox not found / Mailbox is unavailable | The recipient's inbox does not exist or is currently unavailable |
| no mailbox here at that time | There is no user with this email address on the destination server |
| no such user | The specified recipient does not exist on the mail server |
| recipient email address unknown | The server does not recognize the specified email address. |
| user unknown / unknown user | The username is unknown to the server—the address is invalid. |
| said: 550 | Error code 550 indicates a permanent rejection, e.g., because the recipient does not exist or spam is suspected. |
The messages “said: 550” and “said: 553” are frequently used by some servers as bounce messages due to a spam classification.
Filtering by receiving server:
| The email was identified as spam | The mail server identified the incoming email as spam |
| Only registered users may use this system | This means that the server only accepts emails from known or authenticated users—usually relevant for outgoing mail servers |
| recipient address rejected | The recipient address was rejected by the server |
| relay not permitted | The server was used to relay emails, but does not have permission to do so |
Hardware issues with the receiving server:
| error writing message | The mail server was unable to fully process or save the message, usually while caching or forwarding the email |
| message could not be stored | The email could not be stored in the mailbox or in the mail queue |
| relay access denied | The server refuses to relay the email because there is no authorization to do so. |
Mailbox Issues
| mailbox does not exist | The mail server could not find a mailbox with this address. |
| The mailbox has been deactivated / disabled / discontinued | Although the mailbox technically still exists, it is not active. |
| mailbox is full | The recipient’s mailbox is full – the storage limit has been exceeded. |
| mailbox unavailable | The mailbox is currently unavailable – a rather general error. |
| User has exceeded the allowed storage space | User has used up their allocated storage space |
Help & Support
If you have any questions or need help, feel free to contact our service team at any time.