Administrator's Guide - Kerio Software Archive

15.1 Mail Delivery over the InternetPOP3 protocol works on client-to-server basis. Connection is always established by theclient, then requests and responses of the client and of the server take regular turnsuntil the connection is closed. As soon as the client initializes the connection and issuccessfully authenticated by name and password, it is possible to work with the email(download it to the client, delete it, etc.).Under usual circumstances, Kerio MailServer works as a server. If, however, it downloadsemail from remote POP3 accounts, it can also work as a client.POP3 protocol is quite obsolete. The protocol can download email to a client applicationand can work with merely one folder (INBOX). This means that any message moved toanother folder would disappear since moved out of the only folder available. And theother way round. If a user can access multiple folders and moves a message from Inboxto another one, the message cannot be uploaded to the client application. Therefore, it isgenerally recommended to use IMAP, a more modern protocol. Advantages of the IMAPprotocol can be seen in the comparative table 15.1.The only advantage of this protocol might be low demands on server’s disk space. Usersdownload their email to their local disks and there it is possible to sort messages infolders, remove items, etc. Therefore, POP3 accounts are used especially for freemailservices where users have mailboxes with capacity of a few megabytes and download theiremail to their local disks regularly. Another advantage is the good availability of offlinetransactions which can be used if connection to the Internet is time-limited. Nowadays,however, most of email clients work well in their offline modes both with POP3 and withIMAP accounts.IMAPIMAP (Internet Mail Access Protocol) is an Internet protocol used for connections to emailservers, as well as for reading of messages and for other email transactions. IMAP protocolis defined in RFC 3501.In addition to downloading email to users’ local hosts, IMAP protocol enables administrationof email account on the server. It is, therefore, possible to access email accountsfrom various client stations. Unlike POP3, IMAP protocol allows keeping email on theserver and handling it there (reading, removing, sorting to folders). It is also possible tokeep the email stored in the email client. This solution is helpful especially if users havea time-limited Internet connection or can be connected to the server only temporarily orirregularly and need to work with their email offline. Once reconnected to the network,folders on the server and on the client are synchronized.Another difference is that in case of IMAP protocol, email can be handled while items aredownloaded to the local store. In case of IMAP protocol, email headers are downloadedfirst and user can select any of them to be opened as the first. When the message isselected, it will be considered as a high-priority item and it can be read, moved to anotherfolder or otherwise manipulated while the other email is being downloaded.Access via the MAPI interface (MS Outlook)Kerio MailServer enables access to email via the MAPI interface. MAPI (Messaging ApplicationProgramming Interface) is a versatile interface for email transmission, developed139