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Definition of terms


Blacklist are either local or public lists with server addresses and/or private IP addresses and email addresses from servers, computers and senders who are or have been sending spam. Receiving mail servers will most likely not accept email from a server, computer or sender who is listed on such a blacklist. Removal request are handled quickly in most cases, but the responsible network owner usually has to explain to the blacklist administrator what happened and how the problem is solved.

Bounce message

A bounce message is an email generated by your SMTP server, for example under the name Postmaster or System Administrator. There is a difference between a "soft bounce" and a "hard bounce". Soft bounces are temporary problems that may get fixed a little later. A hard bounce is a problem that will for sure not get fixed by itself, for example when you're trying to connect to a server that doesn't exist, or trying to email to a non-existing email address. SMTP server always generate an error codes looking like: 450 or 550 (or any other number, mostly within the range  400 and 599). Some servers may also add an additional error code after it, for example: "553 5.7.1". The error codes that you find in bounce emails are always followed by a textual message. Although error codes from SMTP servers are always the same (they are set out in RFC documents) each server may use different textual error messages for the same type of problem. Also, particular bounce error codes may be caused by different problems.

DKIM or DomainKeys

DKIM (DomainKeys Identified Mail) is a method for associating domain names to email messages. It is used as an extra measurement to avoid spam. Most spammers will try to send spam from email addresses they do not own, either to make it impossible for receivers to block spam based on the email address, or sometimes for fraudulent activities like pretending to be some company and confuse the receiver (for example to ask for private sensitive information). DKIM is an extra measurement used by receiving email servers to stop these activities and only accept email from sending SMTP servers that are allowed to send email for a particular email address. DKIM uses a digital signature that can be validated by recipients by contacting the domain in the sender email address.

Exchange server

An Exchange server is a Microsoft mail server, responsible for both sending and receiving email. Besides email, it can also hold public and private contact lists, calendars, tasks, directories and files. Just like with IMAP servers, email stays on the server by default after a email client has downloaded new email messages.

IP address

An IP address is a unique address that a computer or server is using to communicate with each other over the network. You can compare it with a telephone number. The usual IP address is in the form of 4 octets of 1 to 3 digits (version 4), for example

IMAP server

An IMAP server is a server that is used by email clients (email software), often called the "incoming mail server". IMAP stands for "Internet Message Access Protocol". Unlike a POP3 server, emails will stay on the server by default, after an email client has downloaded new email messages.

Internet Service Provider (ISP)

A business or organization that offers users access to the internet, also called Internet Access Provider.

POP3 server

A POP3 server is a server that is used by email clients (email software), often called the "incoming mail server". POP3 stands for "Post Office Protocol". Unlike an IMAP server, emails are removed from the POP3 server as soon as an email client downloads the new email messages.

Reputation / Sender score

Mail servers use reputation scores in order to lower spam. Each SMTP server has a certain reputation score, depending if the server is known to send spam. Receiving mail servers use this reputation scores to decide whether or not to accept email from a sending (SMTP) server, based on the reputation score. If the SMTP server is known to send spam or is recently being abused by spammers, it will (temporary) have a bad reputation score. Receiving servers that use reputation data will then (temporary) stop accepting email from the SMTP server, until the problem is fixed.

SMTP error codes

Error codes are generated by your email client, for example Outlook or Mozilla Thunderbird. These codes always start with "0x", for example: "0x8004210B" and then give you a textual explanation of the problem. It may also be possible you're not getting any error code, but just the error message itself.

SMTP server

An SMTP server is the server responsible for sending email. All it does is receiving emails from local email clients and then sending it to the recipients receiving mail server.

Trojan Horse

A Trojan Horse is a piece of software running in the background on an infected computer. In most cases it is hidden well and the user is unaware of the existence of the software. A Trojan Horse usually gets installed on a victims computer together with "normal" software; for instance: a user may download and install a computer program or utility and the software will secretly also install extra code that is hidden from the user. Often this piece of extra code is to either spy on the users activities (like monitoring what keys are pressed on the keyboard to get passwords), or it is used to send spam.