550: refused for a header or address problem

What it means

Beyond reputation and content, Yahoo lists a set of narrower policy reasons for which it may permanently defer or refuse mail, giving 550 and 554 as example codes. They are a domain that publishes a null MX record (RFC 7505), meaning it accepts no mail; a header that is too big, such as From, Subject or To; a Date outside the accepted range; a header value such as From or Subject that cannot be decoded; and an email address containing unusual or forbidden characters. Each of these is a fault in how the message was built or how the domain is set up, so the fix sits with the sending software or the DNS, and the message needs correcting before it is sent again.

Common causes

• The sending domain publishes a null MX record, declaring that it accepts no mail.
• A From, Subject or To header is longer than Yahoo accepts.
• The Date header is far in the past or the future, for example because of a wrong server clock.
• A header value cannot be decoded, for example because of broken character encoding.
• An address contains characters such as a comma, $, %, ?, braces or angle brackets.

How to fix it

1. 01Read the reply to see which of these problems Yahoo found.
2. 02Check the sending domain's MX records and remove a null MX if the domain is meant to send and receive mail.
3. 03Send a test message to yourself and inspect the raw headers: the Date should be current, and From, Subject and To should be short and correctly encoded.
4. 04Correct the server clock and time zone if the Date header is wrong.
5. 05Validate addresses where they are collected and drop any that contain forbidden characters.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• SMTP Error Codes (Yahoo Sender Hub)senders.yahooinc.com

Further reading

• MX lookup