IMAP requires three specific "keys" to open the door: the server address, the port number, and the encryption type.
The most frequent reason IMAP fails is a breakdown in "trust" between your client and the server. why-won-t-imap-work
Standard IMAP typically uses port 993 for secure SSL/TLS connections or port 143 for non-secure connections. Using the wrong port—or the wrong encryption method for a specific port—will result in a "Connection Refused" error. IMAP requires three specific "keys" to open the