550 5.7.26
Rejected by the sender domain’s DMARC policy
The From: domain publishes a DMARC policy of p=reject, and your message failed authentication: neither SPF nor DKIM passed and aligned with that domain. The domain owner has explicitly asked receivers to reject such mail, so OxiMail did.
How to fix it
- Send from a host listed in the From: domain’s SPF record (or via a service that is).
- Sign the message with DKIM using a key published under the From: domain.
- Ensure SPF or DKIM aligns with the From: domain (same organisational domain).
- If you operate the domain, check your SPF/DKIM/DMARC DNS records; a DMARC report will show what failed.
550 5.7.1
Refused as spam
OxiMail’s filtering pipeline scored the message as unsolicited or abusive and refused it before delivery. This is a content/reputation decision, not an authentication one.
How to fix it
- Make sure SPF, DKIM and DMARC all pass — unauthenticated mail is weighted as suspicious.
- Check your sending IP and domain reputation (blocklists, recent volume spikes, new/cold IPs).
- Avoid spam-shaped content (deceptive subjects, link shorteners, malformed headers).
- If you believe this is a false positive, contact the recipient directly through another channel so their administrator can review.
550 5.1.1
No such user — the mailbox does not exist here
The recipient address you sent to does not exist on this server. Most often the local part (the bit before the @) is misspelled, the account was closed, or the address was never valid.
How to fix it
- Double-check the spelling of the address, including the part before the @.
- If you copied the address from an old message or directory, confirm it is still current.
- If you are sure the address should exist, contact the recipient through another channel and ask them to verify it with their administrator.
550 5.1.1
This server does not accept mail for that domain
You connected to a server (typically a backup MX) and asked it to accept mail for a domain it does not host or back up. The server will not relay mail for arbitrary domains — that is by design, to prevent open-relay abuse.
How to fix it
- Check the recipient domain’s MX records and deliver to the correct host.
- If you operate a relay, make sure you are connecting to a server that is actually authorised for the recipient domain.
- If you believe this domain should be accepted here, the recipient’s administrator must add it to the server’s accepted domains.
DSN
Your message bounced (returned to sender)
OxiMail accepted the message for delivery but could not deliver it, and after retrying for several days it returned the message to you as a bounce (a Delivery Status Notification, RFC 3464).
How to fix it
- Read the “Diagnostic-Code” line in the bounce — it carries the exact SMTP error from the destination.
- Common causes: the mailbox does not exist (5.1.1), the mailbox is over quota, or a policy rejected it.
- A transient (4.x.x) failure means OxiMail retried for ~5 days before giving up; check whether the destination was reachable.
- Fix the address or the underlying cause and resend.
Still stuck? The recipient’s administrator can look up the rejection in their server logs.