POP3 commands are made up of three to four characters and one or more parameters. When the server and client are connected, they communicate via commands. If the connection is encrypted, port 995 is used. POP3 clients rely on port 110 during the connection process to the mail server through TCP/IP. Each message can only be downloaded by a single POP3 client. The duration of the retrieval process depends on the size of the mail content or attachment. The e-mail content can be opened locally and edited without the client and server being connected. Following this, the e-mails are deleted by the mail server and the connection is terminated.

The e-mails located there are downloaded and saved onto the client’s computer. For this purpose, the client establishes a connection with the incoming mail server on which the necessary POP3 server software must be installed. The Post Office Protocol (POP3) enables e-mails to be retrieved with the help of a client. The response additionally provides information on the success (OK), failure (NO), and syntax errors of the particular IMAP client’s commands. If the reply contains a plus sign at the start, it means the server is expecting additional information about the received command. The reply line starts with an asterisk when information is contained.

In order for the response from the server to be registered at a later stage, the client attaches an identifier to its commands, which the mail server adds to the response. The client communicates with the mail server via text messages, but doesn’t, however, need direct answers to the commands that have been sent out. The connection between the server and IMAP client is established via TCP/IP on port 143 (with a secure port 993 connection). As a result, they can be accessed from anywhere and with numerous clients and always find the same, current database. This way all messages and folder structures stay saved on the server until they are deleted. During this time you can access individual folders and e-mails whose content can be shown on request. This connection is maintained for the whole session. If you set up your account with IMAP, your e-mail client establishes a connection with the server every time you log in. The Internet Message Access Protocol, in short IMAP, is a text-based network protocol that enables access to e-mails located on a mail server.

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