Is my mail server an open relay?

No. Zoho is refusing to relay for an address it is not configured for. Your server’s relay settings are not involved.

No. A 553 is permanent until the domain or From address is set up correctly.

Zoho 553 Relaying disallowed: domain or sender issue

553 Relaying disallowedZoho relaying disallowedPermanent: retrying will not helpFrom Zoho Mail

Example

553 Relaying disallowed

What it means

Reply 553 in RFC 5321 means the mailbox name is not allowed. Zoho uses it with “Relaying disallowed” when it will not handle mail for the address involved. There are two common situations, one on each side.

Inbound, a domain’s MX points to Zoho but the domain is not set up or verified in a Zoho organization, or the address does not exist there. Zoho then refuses to relay the message anywhere. Outbound, a Zoho user sends through Zoho’s SMTP with a From address that does not belong to their account or verified domain, and Zoho refuses to relay it.

Both are configuration issues. Neither is about spam or reputation, and neither is temporary.

Likely causes

A domain whose MX records point to Zoho before the domain was added and verified in Zoho Mail.
A domain that left Zoho while its MX still points to Zoho servers.
An app or device sending through Zoho SMTP with a From address outside the account’s verified addresses.
A mistyped recipient domain that happens to use Zoho.

How to fix it

Check the domain’s MX records to confirm whether it points at Zoho.
If the domain should use Zoho, its admin finishes domain verification and creates the mailbox or alias in Zoho Mail.
If the domain moved away, update MX records to the new provider.
For outbound mail, set the From address to your Zoho account address or a verified alias.

Checks that apply

Your domain or sending IP (fills in the checks below)

MX Lookup Shows whether the domain’s MX still points at Zoho, the most common source of this inbound error.
Domain Health Grades the domain’s DNS end to end, including whether a Zoho verification TXT record and Zoho MX hosts are published.

FAQ

Is my mail server an open relay?: No. Zoho is refusing to relay for an address it is not configured for. Your server’s relay settings are not involved.
Will retrying help?: No. A 553 is permanent until the domain or From address is set up correctly.

All 191 bounce codes