550 5.1.20: multiple From addresses are not allowed without Sender address
The message listed more than one address in the From field but had no Sender field. RFC 5322 requires a single Sender address in that case, so Microsoft 365 rejects it.
Multiple From addresses are not allowed without Sender address
The NDR description in Microsoft's article on 550 5.1.20.
What 550 5.1.20 means
Microsoft generates this NDR when a message has multiple email addresses in the From field but no email address in the Sender field. That follows the message format standard: RFC 5322, section 3.6.2, says that if the From field contains more than one mailbox, a Sender field with a single mailbox must appear in the message.
Microsoft notes that such messages are rare and that many email clients don't support multiple From addresses. In practice, this error usually comes from an application, script or mailing tool that builds the headers itself, for example by putting a comma-separated list where a single From address was expected.
The fix is in whatever builds the message. Microsoft's advice is not to send messages with multiple From addresses at all. If you must, include exactly one address in the Sender field. Check the From value your app or form plugin produces, since a misconfigured field can turn a single address into a list.
Likely causes
- An application or script put several addresses in the From header.
- A form or newsletter plugin joined multiple addresses into one From value.
- The message has multiple From addresses and no Sender header.
How to fix it
If you sent the message
- Use a single address in From. Microsoft recommends avoiding multiple From addresses.
- If several authors are required, add a Sender header with exactly one address.
- Check the configuration of the app or plugin that generates the message, then resend.
If you're the recipient or their admin
- No action needed: the message breaks RFC 5322 and the sender has to correct it.
Frequently asked questions
Can an email have two From addresses?
The standard allows it, but RFC 5322 then requires a Sender header with one address. Microsoft 365 rejects multi-From messages without Sender with 550 5.1.20, and Microsoft notes many email clients don't support multiple From addresses.
Why does my web app trigger 550 5.1.20?
It is probably writing a list of addresses into the From header. Set the From field to one address. If the app needs to show several authors, add a single Sender address as RFC 5322 requires.
Sources
- NDR error code 550 5.1.20 (Multiple From addresses are not allowed without Sender address) (Microsoft Learn)
- RFC 5322, section 3.6.2: Originator fields
Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.
Related codes
- 553 SMTP 553: requested action not taken, mailbox name not allowed
- 501 SMTP 501: syntax error in parameters or arguments
- 550 5.0.350 550 5.0.350: generic error from the recipient's server
- 550 5.1.0 550 5.1.0: sender denied
- 550 5.1.8 550 5.1.8: access denied, bad outbound sender
- 550 5.1.10 550 5.1.10: recipient not found (Microsoft 365)
- 550 5.6.11 550 5.6.11: invalid characters (bare line feeds)
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