What is IMAP?
IMAP (Internet Message Access Protocol) is the standard way an email app or service reads the messages stored in a mailbox on a mail server.
With IMAP, your email stays on the server and each device or app looks at the same mailbox. Read a message on your phone and it shows as read on your laptop. That is the main difference from the older POP3 protocol, which was designed to download mail to one device.
The settings you need
To connect to a mailbox over IMAP, an app needs four things: the server host name, the port, the security type, and a username and password. The usual port is 993 with SSL/TLS encryption. The username is normally the full email address. Your email provider publishes these settings in its help pages.
IMAP only reads mail. Sending goes through a separate protocol, SMTP, with its own host and port.
IMAP and inboxr
inboxr reads incoming email over IMAP. You enter your mailbox's IMAP host, port, username, and password, and a Test connection button checks them before you save. inboxr then checks the mailbox every few minutes for new mail. Any mailbox that supports IMAP and SMTP with a password can be connected.