Getting an error email back immediately after sending one? These are called "Bouncebacks" or Non-Delivery Reports (NDRs). Let's understand what they mean.
Common SMTP Error Codes
If you read the text inside the bounceback email, you will usually find a 3-digit code accompanied by a message. Here is what the most common ones mean:
- 550 User Unknown / No such user here: The email address you are trying to send to simply does not exist. Check for typos in the address!
- 550 Mailbox Quota Exceeded / Mailbox Full: The recipient's inbox is completely full. They must delete old emails before they can receive yours. You cannot fix this from your end.
- 550 Message rejected because SPF check failed: Your domain's SPF record is either missing or misconfigured. Google and Yahoo are very strict about this. See our guide on setting up SPF / DKIM.
- 554 IP Address is Blacklisted: The IP address of your mail server has been flagged for spamming by services like SpamHaus or Sorbs. You must contact support to request an IP delisting.
- 552 Message size exceeds fixed maximum: You attached a file that is too large (usually over 25MB). Use a cloud service like Google Drive or Dropbox and send a link instead!
