Why did my email bounce back?
Decode the bounce message, work out whose end the problem is, and fix the ones you can fix.
A bounce is an automated reply saying your message could not be delivered. Buried in the jargon is a reason code that tells you exactly what went wrong — and crucially, whether the problem is at your end or theirs.
Read the bounce first
Look for a line with a number in it. The first digit is what matters:
The common causes
"User unknown" / "Address not found" (550)
The address does not exist — nine times out of ten a typo, or someone has left that company. Check the spelling character by character before assuming anything else.
"Mailbox full" (552)
Their inbox is over quota, or yours is if you are the recipient. If it is your mailbox, delete large attachments and empty Trash and Junk. Check your usage under cPanel → Email Accounts, where each mailbox shows its storage.
"Message too large"
Most mail servers reject attachments over roughly 20–25 MB, and the encoding overhead means a 20 MB file can arrive as 27 MB. Use a file-sharing link instead — OneDrive if you are on Microsoft 365, or any share link.
"Blocked" / "Spam" / "Reputation"
The receiving server judged the message as spam. Usual causes: attachments it does not like, a link to a flagged site, or your domain missing the authentication records that prove your mail is legitimate. This is the one worth acting on properly — see below.
"Relay access denied"
Almost always a sending device that is not authenticating — the SMTP username and password missing on a phone or in Outlook. See Setting up email on iPhone or iPad.
If your mail keeps landing in spam or bouncing
Modern mail providers check that a domain is authorised to send its own mail. Three DNS records do that job:
- SPF — lists the servers allowed to send as your domain
- DKIM — cryptographically signs your outgoing mail
- DMARC — tells receivers what to do when a message fails the above
These are set up by default on our hosting, but they can be missing if your domain's DNS is managed elsewhere, or broken if another provider also sends mail as your domain (a CRM, a booking system, a marketing platform). If your mail is being rejected or filtered regularly, email us the full bounce message and we will check your records — it is usually a ten-minute fix at our end.
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