IMAP, the Internet Message Access Protocol, is the standard for retrieving and managing email that remains stored on the server. Rather than downloading messages and removing them, an IMAP client keeps the master copy on the server and synchronizes state - read status, folders, flags - so every device sees the same mailbox.
This server-side model is why modern email feels seamless across a phone, laptop, and webmail simultaneously. Mark a message read on one device and it shows as read everywhere; file it into a folder and that folder structure is mirrored on the server for all clients. IMAP supports folders, server-side search, and partial message fetching.
IMAP is the natural counterpart to SMTP: SMTP sends mail, IMAP retrieves and organizes it. It contrasts sharply with POP3, which by default downloads messages to one device and deletes them from the server, making POP3 poorly suited to multi-device use.
IMAP typically runs on port 993 over TLS. For outreach and automation systems, IMAP access is what allows software to read incoming replies, detect bounces and out-of-office notices, and thread conversations, because the mailbox contents remain queryable on the server rather than scattered across downloaded copies.