Error 550

Among all SMTP reply messages, error 550 is surely the most common. It generally comes with a side-message like “550 Requested action not taken: mailbox unavailable” or “550 5 2 1 mail from refused spam site”.

Simply put, a 550 error code means that your SMTP server isn’t able to deliver the sent email to the user because his mailbox does not exist: either the customer has entered it wrong (a typical example is something like mail@otmail.com instead of mail@hotmail.com), or the account has been deactivated and replaced with a new one (but you didn’t know it).

In any case, you must remove this address from your mailing list: keeping sending messages to it will just cause other 550 messages. This is not simply annoying; it is dangerous as well for your deliverability. In fact, it implicitly suggests to ISPs and reciving systems that you don’t really care about your list hygiene – just like spammers: so with time you could be penalized and see your delivery rate decrease.

Of course the best practice here is not simply to erase the address that provokes a 550 error code, but to replace it with a new one whenever possible. How to do that? If you have another way to contact your user – a phone number, Twitter, Skype… – just send him a line to apprise him that his current email account is wrong, and to ask him a functioning one.

STMP Error List

Anybody who sends email has seen them, in one form or another – those SMTP error codes, often returned in bounced email, such as “550 Requested action not taken: mailbox unavailable” or “550 5 2 1 mail from refused spam site.” These are often in response to SMTP commands that have ‘gone wrong’ between your email server that sent the email, and the receiving email server that is unable to deliver it (or refuses to deliver it) for some reason. But what exactly do they mean? And why should you care? (‘SMTP’ stands for Simple Mail Transfer Protocol.)

First, here is a list of SMTP codes and error codes, and what each message is supposed to say and mean:

211 – A system status message.
214 – A help message for a human reader follows.
220 – SMTP Service ready.
221 – Service closing.
250 – Requested action taken and completed.
251 – The recipient is not local to the server, but the server will accept and forward the message.
252 – The recipient cannot be VRFYed, but the server accepts the message and attempts delivery.

354 – Start message input and end with .. This indicates that the server is ready to accept the message itself

421 – The service is not available and the connection will be closed.
450 – The requested command failed because the user’s mailbox was unavailable (such as being full). Try again later.
451 – The command has been aborted due to a server error. (on their side)
452 – The command has been aborted because the server has insufficient system storage.

Unfortunately, many receiving systems seem to mix and match these error messages, rather than adhering to the prescribed code messages.