SMTP error 517
Non-standard label: bad sender attribute, or a policy block.
Updated Jul 1, 2026
SMTP 517 is not a standard reply code. RFC 5321 defines no 51z codes, and a second digit of 1 means informational, which contradicts the permanent-failure 5. Vendors disagree on what "517" means: some tie it to a malformed sender mail attribute (often Exchange or Active Directory), others to the recipient server blocking the sender on policy. Read the full server response before troubleshooting.
Read the full server text, then fix the sender address attribute or check your reputation.
What it means
There is no 517 reply code in RFC 5321. The second digit carries meaning: 0 syntax, 1 information, 2 connections, 3 and 4 unspecified, 5 mail system. A leading 5 means permanent failure, but a 1 in second position means informational, so a "permanent informational" code is a contradiction and the RFC defines no 51z codes at all. The closest standard code is 521 ("server does not accept mail"). If you literally see 517, treat it as a vendor or appliance string and read the full text the server returned.
Common causes
One framing pairs 517 with "Problem with senders mail attribute, check syntax", common in Exchange or Active Directory where a user object's mail attribute (the primary SMTP address) is missing, malformed, or has invalid characters. The originating server rejects before the message reaches the recipient.
Other vendor docs describe 517 as the receiving server blocking or suspending the sender on policy (spam, blocklist, or phishing suspicion), sometimes paired with different enhanced codes.
Because 517 was never standardized, different systems attach different explanations to the same three digits.
How to fix it
- Capture the full server response
The three digits alone are not enough. Copy the entire line and note which server emitted it. The text tells you whether it is an address attribute, a policy rejection, or a header issue.
- Validate the sender address
Confirm the MAIL FROM and From address is a valid RFC 5321 address: no spaces, no leading or trailing dots, no invalid characters, and a real domain.
- In Exchange or Active Directory, check the mail and proxyAddresses attributes
Ensure the mail attribute holds a valid SMTP address and that proxyAddresses has exactly one primary SMTP: entry. Remove stray whitespace or non-ASCII characters.
- Inspect message headers
Malformed From, Sender, or Return-Path headers can trigger sender-attribute rejections. Send a test through a tool that shows raw headers.
- Check whether the recipient is blocking you on policy
If the text mentions spam, reputation, or a blocklist rather than syntax, this is a reputation issue. Check your domain and IP against major blocklists and review recent complaint and bounce rates.
Common questions
Is 517 a real SMTP reply code?
No. RFC 5321 defines no 51z codes; a second digit of 1 (informational) conflicts with the permanent-failure 5. It is a vendor or appliance string, defined differently by different vendors.
What is the difference between 513, 515, and 517?
None are RFC codes. Roughly: 513 is described as an address-format problem, 515 as a recipient that does not exist, and 517 as either a bad sender attribute or a policy block. The full server text is the only reliable signal.
How do I fix a 517 sender mail attribute error in Exchange?
Check the affected user's mail and proxyAddresses attributes for a valid, single primary SMTP address with no stray whitespace or invalid characters.