POP3 and IMAP4 are two different protocols (methods) used to access email.
Of the two, IMAP4 is the better option – and the recommended option – when you need to check your emails from multiple devices, such as a work laptop, a home computer, or a tablet, smartphone, or other mobile device. Tap into your synced (updated) account from any device with IMAP.
POP3 downloads email from a server to a single computer, then deletes it from the server. Because your messages get downloaded to a single computer or device and then deleted from the server, it can appear that mail is missing or disappearing from your Inbox if you try to check your mail from a different computer.
SpinOffice CRM supports both POP3 and IMAP4.