IMAP, explained
IMAP (Internet Message Access Protocol) lets an email client read and organise mail that is stored on the mail server. Because the messages stay on the server, reading, moving or deleting an email on your phone is reflected on your laptop and in any other app connected to the same mailbox.
That is what makes IMAP useful for connecting a mailbox to a CRM. The CRM connects to the mailbox with the server address and the account's credentials — often an app password rather than the main one — and reads the conversations with your leads and customers, so they can be matched to the right records without anyone forwarding emails by hand.
IMAP is different from POP3, an older protocol that downloads mail to one device and usually removes it from the server, which is why POP3 does not suit shared or multi-device use. When you connect a mailbox over IMAP, use the secure port your provider documents and a dedicated app password where your provider offers one. If that password changes, update it in every connected app, or the sync stops until you do.