Simple Mail Transfer Protocol has a number of associated error codes, some are specific to mail delivery whilst others concentrate on the connection side of things. Here are some of the more common errors and what you can to do to help resolve the issue:-
Error Codes Starting With A 1 – Server Related
SMTP Error 101 – Can’t Connect To Server. If you see this error when attempting to send e-mail the chances are that you have specified the SMTP server incorrectly. Double check the server, especially the domain prefix, some common examples include mail. smtp. out. mailout. If you are sure you have the correct server address then make sure you are using the right port number, port 25 is standard but this can cause its own problems.
SMTP Error 111 – Connection Refused. Similar to 101, the solution is also the same.
Error Codes Starting With A 4 – Mail Delivery / Routing Problems
SMTP Error 421 – Service Unavailable. This normally relates to the remote server being down, dpending on how things are configured in will usually result in the transmission being automatically retried later on. Codes 441 and 442 are due to similar circumstances.
SMTP Error 447 – Message Time Out. You are trying to hammer a large mailing list aren’t you? If you get this error first of all try sending the mail in smaller batches.
SMTP Error 450 – The requested action has been rejected. This is the tell tale error that your e-mail address has been blacklisted, the recipients server is actively rejecting the e-mail you are attempting to deliver.
Error Codes Starting With A 5 – Addressing/ SMTP Syntax Errors
SMTP Error 500 – Syntax Error. Usually your anti virus will be to blame. Error 501 can also occur.
SMTP Error 503 – Bad Sequence. 99 times out of a hundred this is an authentication issue, the SMTP server is expecting a certain type of authentication but the mail client is hitting it with a different set of commands.
SMTP Error 512 – DNS Problem. The mail server for one of your recipients can’t be found, double check to make sure that you haven’t put in an invalid domain.
SMTP Error 541 – Recipient has rejected your mail. You mail has been rejected, most likely due to a spam filter.
SMTP Error 550 – Recipient doesn’t exist. The receiving server is letting you know that it doesn’t have the address you have specified, in this instance you’ll need to find another way of contacting the person to get their correct e-mail address.
SMTP Error 552 – Exceeded Storage Allocataion. You’ve attempted to send a message that is too big, try removing/zipping attachments. To be honest this is rare with modern e-mail accounts but some smaller ISPs are still quite restrictive with mailbox and individual mail limits.
SMTP Error 554 – Mail delivery Failed. This is a permanent error and of all the codes is the one you least want to see. It is generally a very good inidcation that you have been blacklisted.
There are several more codes, any beginning with a 2 are generally success/status updates and are nothing to worry about. The problem with some of the codes above are that they may require some input from your mail provider, be that your ISP, a website host or a 3rd party mail provider. A large number of these have very little understanding of the mail process and can leave you going around in circles. It’s for that reason that we recommend using the SMTP2Go server, their whole business is mail delivery and will always undertand any issues you may be having.