5.5.0: SMTP protocol problem, unspecified
Something went wrong in the SMTP conversation between the servers that the receiving side did not describe with a more specific protocol code.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.5.0
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
5.5.0 Other or undefined protocol status
What it means
X.5.x codes concern the mail protocol itself: commands, their order, their syntax and their arguments. With detail 0, the server reports a protocol issue without classifying it. The registry lists it with a long range of reply codes, including success replies, so it turns up in many contexts; RFC 4468, for example, shows 554 5.5.0 when a client goes on to send a message after every recipient was refused. 5.5.0 means the command failed for good in this session; a 4.5.0 means a temporary protocol state. When a whole message bounces with it, the sending software is the place to look.
Common causes
- A command sent at a point where the server did not expect it.
- A client that kept going after all its recipients were refused.
- Sending software that does not follow SMTP closely.
How to fix it
- 01Read the text after the code; it usually names the command that failed.
- 02Check the recipient replies earlier in the session, and fix those first if all were refused.
- 03Update or reconfigure the sending software, or test with a standard mail client.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.5.1Invalid SMTP command.The sending software issued an SMTP command that the server does not support, or sent it at the wrong point in the conversation.
- 503Bad sequence of commands.The client sent a command at the wrong point in the conversation, for example message data before any recipient was accepted, or mail before logging in.
- 554Transaction failed.The server permanently refused the transaction, often after receiving the message data or straight away when the connection opened.
5.5.0 at other providers
All 105 standard codesEvery code in the wikiPaste your own bounce