imap: Mail servers
IMAP, short for Internet Message Access Protocol, is a standard network protocol used to access, manage and synchronize email messages on a remote mail server. Unlike older protocols such as POP3, where emails are often downloaded and then deleted from the server, IMAP ensures that the messages remain on the server. This makes it possible to access the same mailbox from multiple devices – such as a laptop, smartphone or tablet – including folder structure, read status, labels and other characteristics. The protocol is designed for use in modern, network-based email environments where consistency and synchronization are important.
The current and most recent specification of IMAP is laid down in RFC 9051, published on August 23, 2021. This RFC introduces IMAP4rev2, the successor to IMAP4rev1, which was laid down in RFC 3501 in March 2003. RFC 9051 officially obsoletes RFC 3501, which means that the new version formally replaces the previous one as the valid specification of the protocol. Both RFCs were released by the IETF (Internet Engineering Task Force), the main organization that develops and maintains internet standards. The IETF works on the basis of open collaboration between technical experts, and the publication of an RFC (Request for Comments) is a crucial step in the standardization process of network protocols.
RFC 3501, also known as IMAP4rev1, was written by Mark R. Crispin, an important figure in the development of email protocols. Crispin was affiliated with the University of Washington in Seattle, an institution with a long history in the development of email software. The University of Washington was responsible for, among other things, the original version of the email client Pine, which would later grow into Alpine. Crispin's work on IMAP began as early as the 1980s, and RFC 3501 was an important milestone in the further standardization of the protocol. This version replaced earlier specifications, including RFC 2060, and introduced a robust, extensible basis for email access via the internet.
One of the core characteristics of IMAP is that it is a protocol for access and management of email messages on a server. The protocol describes how an email client – such as Alpine, Evolution or a mobile app – can connect to a mail server, browse mailboxes, retrieve messages, apply labels, and manage folders. However, IMAP itself is not intended for sending email. SMTP (Simple Mail Transfer Protocol) is usually used for sending. This distinction is important: IMAP handles the receiving and management side, while SMTP is responsible for the sending side. The combination of these protocols makes a complete email workflow possible.
An important technical improvement in RFC 9051 compared to RFC 3501 is the expansion of the supported message size. While IMAP4rev1 had a limit of 32 bits for the size of an email message, IMAP4rev2 now supports messages up to 63 bits. This means that much larger files – such as attachments of several gigabytes – are technically possible without the protocol becoming the limiting factor. This expansion is a response to the growing need to send and store large files via email, and makes IMAP more future-proof in a time when cloud storage and bandwidth are increasingly less of a bottleneck.
In addition, RFC 9051 introduces explicit support for UTF-8 in various parts of the protocol. This means that both mailbox names and message subjects can be displayed correctly in non-Latin character sets, such as Cyrillic, Arabic or Asian scripts. This improvement enhances the international usability of IMAP and reduces problems with character encoding that used to often lead to unreadable text in email subjects or folder names. UTF-8 support is an important step towards a more inclusive and global email infrastructure.
For users, IMAP means that their email experience is consistent, regardless of the device they use. If you mark a message as 'read' on your phone, you will see it that way on your laptop. If you create a folder or archive an email, it happens on the server – and therefore visible to all devices connected to that server. This stands in sharp contrast to POP3, which often takes a 'download-and-delete' approach and is less suitable for multiple devices. IMAP is therefore the standard choice for modern email services, such as Gmail, Outlook.com and business email solutions.
On the server side, various IMAP implementations are available, of which Dovecot is one of the most used. Dovecot is an open source IMAP and POP3 server for Linux and UNIX-like systems, designed with an emphasis on security, speed and simplicity of configuration. It is widely used in personal, business and educational environments. Dovecot supports both IMAP4rev1 and the newer IMAP4rev2 functionality where applicable, and is known for its low resource usage and high stability. The software is written with security as a priority, which is essential for a mail server that is continuously exposed to network attacks.
On the client side, various email programs offer support for IMAP. Alpine, a text-based email client, is a modern successor to Pine and is often used in terminal environments. It is designed for remote use via SSH and supports IMAP connections for access to remote mailboxes. Alpine is easy to use and suitable for users who prefer a lightweight, quick-to-operate mail client without a graphical interface. It also offers integration with GnuPG via a separate tool, with which emails can be encrypted and signed according to OpenPGP standards.
Another example is GNOME/Evolution, the default mail client for the GNOME desktop environment. Evolution supports not only IMAP, but also a wide range of other protocols and services, such as CalDAV, CardDAV, WebDAV, Microsoft Exchange and Novell GroupWise. It is a complete personal information manager (PIM), which manages not only email but also calendar, contacts, tasks and RSS feeds. Evolution uses IMAP to synchronize email messages with the server, and offers a rich user interface for desktop users. It is a widely used solution in business and government environments where integration with other systems is important.
The configuration of IMAP in email clients varies per program, but generally follows the same pattern. The user must specify the server name (for example imap.example.com), the port (usually 993 for IMAPS, i.e. IMAP over TLS), and the security settings. Authentication usually takes place via username and password, sometimes in combination with two-factor authentication or OAuth2 for extra security. Once the connection is established, the client can retrieve the mailbox structure and start synchronizing messages.
In summary, IMAP is a crucial protocol in modern email infrastructure. It allows users to keep their email consistent and up-to-date on multiple devices, while limiting the management burden for administrators by keeping everything on the server. With the introduction of IMAP4rev2 in RFC 9051, the protocol has been refined for the needs of the 21st century: larger messages, better language support and a future-proof architecture. The ongoing development of IMAP, supported by tools such as Dovecot and clients such as Alpine and Evolution, ensures that it continues to meet the requirements of security, reliability and ease of use.