Ralf Zimmermann SIEGNETZ.IT GmbH

All SMTP Error 101 errors usually point to a configuration problem, such as an incorrectly spelt SMTP server, or an IP address that does not exist, or an SMTP port that does not exist or which the recipient will not accept SMTP connections on, or some other process is already using the default SMTP port, port 25.

All SMTP Error 111 errors usually point to an inability of your server to communicate with the remote SMTP server (either the recipient’s SMTP server or your ISP’s SMTP server) or to a Linux/SMTP software configuration problem, typically /etc/hosts not being world readable, or a newly installed or reconfigured firewall preventing connection to the remote SMTP server, or incorrect hostnames and/or domains (e.g. does your sending hostname match your IP address in a reverse lookup?), or exim not running. Telnet and logs should help you home in on the problem.

SMTP Status 221 is often misconstrued as an error condition, when it is in fact nothing of the sort. The mail server is simply telling you that it has processed everything it was given in this particular session, and it is now going back into waiting mode.

Because SMTP status 221 is often misinterpreted, with some mail servers the Network Administrators have changed the default text of SMTP Reply 221 to something more meaningful and less alarming. For example, a typical SMTP reply 221 might say “221 Goodbye” or “221 Closing connection”, or the most irritating one we’ve seen “221 Bye”, Arrrgghh–can you blame anyone for thinking there might be a problem? Of course not! So some Network Administrators are these days being quite imaginative by changing the default text of SMTP reply 221 to more user friendly messages like : “221 Thank you for your business”(I love that one!),or “221 All messages processed successfully in this session, SMTP connection is closing”.

SMTP Status 221 is often misconstrued as an error condition, when it is in fact nothing of the sort. The mail server is simply telling you that it has processed everything it was given in this particular session, and it is now going back into waiting mode.

Because SMTP status 221 is often misinterpreted, with some mail servers the Network Administrators have changed the default text of SMTP Reply 221 to something more meaningful and less alarming. For example, a typical SMTP reply 221 might say “221 Goodbye” or “221 Closing connection”, or the most irritating one we’ve seen “221 Bye”, Arrrgghh – can you blame anyone for thinking there might be a problem ? Of course not ! So some Network Administrators are these days being quite imaginative by changing the default text of SMTP reply 221 to more user friendly messages like: “221 Thank you for your business” (I love that one!), or “221 All messages processed successfully in this session, SMTP connection is closing”.

250 is effectively a status code rather than an error code – there is no such thing as an SMTP error 250.

most common action for ISP Mail servers – the recipient will see your ISP in the mail header as one of the first hops on the way to the recipient’s email system).

SMTP Error 251 is therefore more of an informational message for technicians tracking how a message reached its destination.

deliver the message.

There are sometimes circumstances where an email address appears to be valid but cannot be verified as definitely valid during the SMTP session between the sending server (your server) and the next server to accept your message. This can happen for example in very large corporation where the first email receiving server might only be an email exchanger server, a gateway server to the eventual server which holds the user mailboxes and which can verify if the intended recipient exists in that organization. When this happens the gateway server will reply with an SMTP Error 252 telling your sending server that it cannot verify the user part of the email address, that the domain part is OK, and that it will forward your email to a server which can do the checking and eventually deliver to the user mailbox if it exists.

Therefore, on receiving an SMTP Reply 354 the sending server should send the body of the message to the receiving server and indicate the end of the message body with <CRLF>.<CRLF> (note the full stop between the two Carriage_Return-Line_Feed’s).

If you experience 420 errors only with specific recipient then it is quite likely that the recipient’s antispam firewall does not like your server, your server’s external IP address, or that your server’s HELO command uses an outbound identification that does not match your server’s external IP address (check that your sending domain’s DNS is set up correctly). In an ideal world a well behaved recipient server should really be issuing your GroupWise server with a 554 error rather than timing out and causing the GroupWise GWIA to fault with a 420 error.

If you experience too many 420 errors with all email communications, then you have a physical communication problem somewhere. This could be your server’s network card, the network point that your server is plugged into, your switch(es), your router(s), your firewall, or your Internet line – problems caused by routers with different MTU sizes is a classic issue. Unless the logs of all those various problem points can give you an instant answer, the only way you will get to the bottom of the problem is to use a packet tracing and inspection program like Ethereal or Wireshark, its successor, if you’re running GroupWise on a Windows or Linux server; on NetWare your only choice is PacketScan which you can get here http://support.novell.com/docs/Readmes/InfoDocument/2967287.html. In the final analysis, if the tracing of packets, and the changing of hardware does not help then do not discount a slightly faulty hard disk being the cause of all your problems (even if your RAID controller or your hard disk testing software does not detect any problem!).

421 can be caused by many things but generally indicates that the mail server which returns this status code is currently unavailable but may be available later.

For example, the server administrator may have stopped the mail service to troubleshoot a problem, or the mail server is right in the middle of rebooting, or the mail server is currently processing too many incoming messages or incoming requests, etc... Note : “Mail Server” in this case can be any of the mail servers on the message’s route – the sending server (your server), the ISP SMTP server, or the recipient’s mail server.

Clearly, if you repeatedly receive an SMTP status 421 then the problem is no longer of a transient nature and you need to investigate or inform the relevant network administrator, ISP tech support, or the recipient.

SMTP Response 421 can also be received as a result of your message server sending an email where the total number of TO, CC, and BCC users results in a number of simultaneous SMTP connections that is in excess of the number of connections your ISP or SMTP service allows. A typical error message for this situation would be : “421 Too many concurrent SMTP connections from this IP address; please try again later”. Typically, when this happens your server will have sent some of the messages (note that for all servers, each email sent by a user always gets broken down into individual separate emails to each of the recipients in the TO, CC, and BC fields), and will automatically retry a little later to send the remaining messages.

However, SMTP response 422 can also be received if the email being sent is larger than the incoming emails size limit in operation at the recipient’s mail server (particularly when that recipient’s mail server is Exchange Server).

In general SMTP Error 450 is a transient error at the remote end (the destination), or at one of the routers or servers en route to the remote end, and should induce your mail server to retry after it’s preset retry interval. Example of an SMTP Error 450 reply message: “450 Please try again later”, or a classic Novell GroupWise 450 status message: “The message that you sent has been delayed. The reason given for the delay: 450 Host down (relay.clara.net)”.

SMTP Error 450 is often followed by a second SMTP error code to refine the reason for the email not reaching its destination. For example: “SMTP Error 450 5.2.3 Msg Size greater than allowed by Remote Host”. When that is the case and If the error message is not as clearly worded as in this example, then simply search this document for the secondary error code. In this case searching this document for SMTP Error 523 or SMTP Error 5.2.3 would yield an explanation identical to the wording above.

If this error keeps occurring to the point that it has effectively lost its transient nature and has become... frequent (!!), then the problem is at your end and you should check your own mail server (if you email out of a corporate network), communications on your side (router, server network card), or inform your ISP if your mail server relays through your ISP or if you are a home user emailing out through Outlook, Outlook Express, Windows Mail, or similar email program.

Examples of typical SMTP Error 451 return messages: “SMTP error 451 Unable to complete command, DNS not available or timed out” or “451 Domain of sender address does not resolve” or “451 Error getting LDAP results in map”, or “451 4.7.1 Greylisting in action, please come back in 00:02:00 [minutes]” or “The message that you sent has been delayed. The reason given for the delay : 451 Temporary local problem - please try later.”.

With the original SMTP standards having been invented before spam became the scourge of the Internet, there are no SMTP error codes dedicated to anti-spam errors. As a result, SMTP Error 451 is now increasingly also used to indicate that a message has been rejected by the remote server because of anti-spam measures. A typical error might be, for example: “SMTP error from remote mail server after end of data, host <host_address>: 451 <ip_address> has recently sent spam. If you are not a spammer, please try later.”.

If all anti-spam related SMTP 451 errors are as descriptive as the one above, then the error itself will tell you what you need to do. As a general rule, however, you will most times need to take some measures to have either your server, or your ISP’s server, taken off some Internet blacklist used by the recipient.

Note that commands that are recognized, but not implemented, are handled by different status messages (see 502 and 504).

Note: A "500 unrecognized command" server response is often a case of antivirus software and/or firewall interfering with incoming and/or outgoing SMTP communications. Read your antivirus / firewall software documentation thoroughly to solve the problem.

Examples of SMTP Error 500 error messages: SMTP Permanent Error: 500 Access Denied By Port Access” or “SMTP Error 500 Line too long.

For example, the following email address will definitely give an SMTP Error 501 with most mail servers, happy\[email protected], as “\” is not allowed in email addresses, which makes this email address invalid.

In the vast majority of cases SMTP Error 501 is caused by invalid email addresses, an invalid domain name recipient, or a Unix / Linux SEND MAIL command which does not follow the established standards. For example, a typical return error message might be: "<remote-server-ip-address> does not like recipient. Remote host said: 501 Invalid Address".

In cases where the error is not caused by an invalid email address, or by the failure to assign a valid email address to the mandatory "From" property, an SMTP Error 501, particularly if repeated, can be indicative of communications problems, such as a noisy line, intermittent drops in network connections, etc...

This can often be caused by a drop in network connection just as your server was sending a command, resulting in the ISP’s server not receiving it and consequently not understanding the command that followed it.

SMTP Reply Code 503 is nowadays more often an indicator that the SMTP server you are trying to use requires authentication and you tried to send a message without authentication (username + password). This SMTP Error 503 is permanent in that the SMTP server will not log any errors in its log and it will not retry – you will have to resend the email using authentication. Example of such an error : “SMTP Error (state 13): 503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server”.

For example, an often encountered SMTP Error 504 is : “504 Need to authenticate first”

If the email was addressed internally, then it means that the addressee, as written in the email’s TO, CC, or BCC fields, does not exist in your organization’s email system.

If the email was addressed externally, then the recipient’s email address was misspelt.

If the email was addressed internally, then it means that the addressee, as written in the email’s TO, CC, or BCC fields, does not exist in your organization’s email system.

If the email was addressed externally, then the recipient’s email address was misspelt.

The first check you should perform to resolve a 5.1.2 reply code is to check all the recipient email addresses for incorrect domain names (misspelt domain names, or, maybe, totally non-existent domain names) – remember, error code 512 is very specifically an error with the domain name of one of the recipient email addresses. You can call the recipient(s) or use the WHOIS tool of The Ultimate Troubleshooter. If all the recipient email addresses check out as regards the domain part of the email addresses, then one of the servers on the way to the recipient(s) has DNS problems – usually this will be one of the first 2 servers in the chain, your own mail server (or your network) or your ISP’s mail server.

Examples of typical SMTP error 512 messages : “5.1.2 - Bad destination host 'DNS Hard Error looking up domain”, or “SMTP Error 550 5.1.2 Host unknown – host cannot be found”, or how about this fantastically informative error message “5.1.2 The message could not be delivered because the recipient's destination email system is unknown or invalid. Please check the address and try again, or contact your system administrator to verify connectivity to the email system of the recipient.”.

mail server) is usually symptomatic, in an Exchange + Outlook environment, of the user’s Outlook Contacts having been imported from another system or PST and where some of the addresses are not defined correctly. Or, in any environment it is simply that the end-user simply did enter the email address completely wrongly, such as copying it from a website and not replacing “at” with “@”, e.g. : John.DoeatUCLA.edu (which should have been [email protected]), or [email protected]” (“, quotes, is not allowed in email addresses and is often included in error as a result of copying and pasting an email from somewhere).

The user should check all the recipient addresses in the email, including those that were inserted from Contacts.

total size of the message you have sent (ie: message + all of its attachments) exceeds the size limits on the Recipient’s server. Many companies implement the good practice of configuring their servers with limits on the size of emails they can receive to prevent their systems running out of space as a result of a spam attack where the spam emails contain large attachments, or as a result of valid but not very technically savvy senders sending enormous scans (through not knowing that scanning at 1200dpi rather than the usually perfectly usable and acceptable 300dpi, will create humongous attachments).

Check the size of the email you sent, and, specifically, the size of the attachments you included, and consider splitting your email into smaller emails. If that does not work, check with the Recipient the maximum size of email they can receive, and if that is still prohibitive then consider FTP arrangements between you and the recipient.