SMTP error 5.1.3 Bad destination mailbox address syntax

What SMTP error 5.1.3 means

The address is not a valid address. Not wrong, not dead: malformed. Something in the string breaks the rules for how an address must be written, so it is refused before anyone tries to deliver anything. It usually shows as 501 with wording about a syntax error, and the offending character is nearly always invisible on screen — a trailing space, a stray comma, a curly quote pasted out of Word.

Address status
Sent by Whichever server first parses the address — often your own, sometimes the recipient's
Occasional

What SMTP error 5.1.3 Bad destination mailbox address syntax actually means

Every other addressing failure assumes the address is at least well formed. This one does not get that far. A server parses the recipient string, finds it does not conform, and rejects the command outright, which is why the reply is a 501 syntax error rather than a delivery verdict. Any hop can spot it, so your own client, your gateway or the far end may be the one objecting. In practice the address was rarely typed by a person one character at a time. It came from a copy and paste, a mail merge or a CSV import, and it brought punctuation with it.

The destination address was syntactically invalid. This can apply to any field in the address. This code is only useful for permanent failures.
— RFC 3463

How 5.1.3 appears in a bounce

A server reporting this condition sends it alongside a three-digit reply code, and a bounce prints the two together. The registry lists 501 5.1.3 as the pairing for this status.

The leading 5 is the server's verdict rather than part of the code's identity: it marks this as a permanent failure, which means the sending server has given up and returned the message.

What causes SMTP error 5.1.3 on a business phone system

A trailing comma or semicolon left by a copy and pasteLifting recipients out of a document carries the separators along with the addresses. Outlook separates with semicolons and much of the rest of the world uses commas, so a pasted list often ends up with one glued to an address.
Smart quotes and other characters from a word processorWord converts straight quotes to curly ones and hyphens to en dashes as you type. An address that has been through a Word document or a formatted PDF carries those substitutes, which are not permitted characters. On screen it looks normal.
A space inside the addressUsually at the start or end after a paste, occasionally in the middle where a long address wrapped across two lines and was reassembled with the line break turned into a space. The parser stops there.
Two @ signs, or noneProduced by joining a name and a domain in a script or spreadsheet formula where one already contained the @, or by a mail merge field filling in the wrong column. An address needs exactly one.
A display name that was never turned into an addressJane Smith <jane.smith@example.co.uk> pasted into a field expecting the address alone, or an import that put the friendly name in the address column. Legal in a header, illegal in a field wanting only the address.

How to fix SMTP error 5.1.3

Retype the address by hand into a fresh messageThe fastest diagnostic there is. If a hand-typed address sends and the pasted one does not, the problem is a character you cannot see, and you need not identify which one before getting the message away.
Paste through a plain text editor to strip the formattingNotepad, or any editor with no autocorrect, discards smart quotes and formatting artefacts. Paste in, copy back out, use that version. Turning off automatic quote substitution in Word stops it recurring.
Check the separators on multi-recipient sendsConfirm the list uses the separator your client expects and that no address carries one at its end. Where a list came from a spreadsheet, rebuild it in the To field rather than pasting the block in one go.
Validate the column before importing contacts in bulkBefore a CRM or mailing list import, filter for rows with a space, with no @ or with more than one. Doing it once in the spreadsheet is quicker than tracing the bounces those rows produce later.
Look at the address in raw form rather than friendly formOutlook shows a display name and hides the underlying address. Open the recipient's properties so you are inspecting what will actually be sent rather than the label sitting on top of it.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about SMTP error 5.1.3

The address looks completely normal to me. What am I missing?

Almost certainly a character that does not render distinctly: a non-breaking space, a curly apostrophe, or a trailing space after the final letter. Walk through the address with the arrow keys, or paste it into a plain text editor where the extra character usually becomes obvious.

Why did our mail merge bounce for a handful of recipients and work for everyone else?

Because a handful of rows in the source data are malformed and the rest are clean. Merges do not validate addresses, they insert whatever the column holds. Filter the source for rows with a space, no @ or more than one, correct those, and rerun for the affected recipients.

Should I report this to the recipient's IT department?

Not usually. Their server generally never saw the message, because the malformed address was caught locally or at the first hop. The fault is in the address string held on your side, so the fix belongs with whoever or whatever produced it.

Related SMTP status codes

Source. SMTP error 5.1.3 Bad destination mailbox address syntax is defined in RFC 3463 and registered in the IANA SMTP Enhanced Status Codes registry. The causes and fixes above are drawn from our own experience supporting UK business email systems.

All Email delivery & SMTP codes