RFC defined SMTP Status Error Codes

SMTP protocol specific response code design provides some useful (numeric) codes used for reporting mail (delivery status) system errors (Protocol a method of sending and receiving data).

SMTP is a simple ASCII protocol. SMTP is a protocol governing electronic mail transmission and reception (sending and receiving emails between mail servers).

ASCII is a standard developed by the American National Standards Institute ANSI to define how computers write & read characters --used by most OS (but not by Windows NT)

Operational Sequent of SMTP Simple Mail Transfer Protocol

How two MTAs (Message Transfer Agent) communicate with each other using a single TCP connection?

1. Email is delivered by the source host (sender) establishing a TCP connection to port 25 of the (recipient) destination host and wait...
    
2. An email daemon speaking SMTP simple ASCII rfc 821 listening at port 25 on the (recipient) destination hose identify itself and tell the (sender) whether it will accept mail....
    
3. *If not* Server-time-out--the sending mail server terminate the connection and try again later, after a predefined undeliverable attempts--any undeliverable message will be returned with an error report containing the first part (message headers) to the sender.
    
4. If the (recipient) mail server is willing to accept the mail, the (sender) mail server sends the email address of the recipient...
    
5. If it is a valid recipient email address, the (recipient) mail server gives a go-ahead signal...
    
6. Message is sent and acknowledged.

*If not*--There are no standard mechanism for reporting mail system errors except for the limited codes consisting of three numbers offered by SMTP with specific text descriptions sent in mail messages.

Because of its limitations, the majority of each code delivery report may indicate several other error conditions as well as different mail server using their own text description.

The first number generally tells whether the server
accepted the command and if it could manage it.

2nd number - The server has completed the task successfully.

3rd number - The server has understood the request but require further information to complete it.

4th number - The server has encountered a temporary failure. If the command is repeated without any change,
it might be completed.

5th number - The server has encountered an error.

The second number gives more information and its six possible values are:

0 A syntax error has occurred.

1 Indicates an informational reply, i.e. to a HELP request.

2 Refers to the connection status.

3 Unspecified.

4 Unspecified.

5 Refers to the status of the mail system
   as a whole and the mail server in particular.

The third number show more graduations of the mail transfer status to return a more detailed report as detailed in the ESMTP server response codes defined in RFC 821 and later extensions.

Example of an older SMTP response text description for the first number.

2.x.x    Success (The server has completed the task).

4.x.x   Persistent Transient Failure
(The server has encountered a temporary failure).

5.x.x   Permanent Failure
(The server has encountered an error).

The number 5.x.x at the beginning of the specified SMTP protocol response code is about the only thing you can go by, because different mail server can use their own text description with the second and third number.

Example of new ESMTP response

Extended SMTP (ESMTP) consists of several extensions to the basic SMTP protocol. All of the extensions are optional.

SMTP servers may support any extension that it chooses or none at all. See: Definition of ESMTP

501 5.5.4  Syntax error

501 Syntax error represent the enhanced SMTP (ESMTP)status code

5.5.4 represent the old SMTP status code