A script for testing IMAP logins. Simply edit SSL_ENABLED to determine if you need SSL or not, IMAP_PORT if you aren't using a standard IMAP or IMAPS port, USERNAME, PASSWORD, and IMAP_SERVER to all suit your own setup. Then run the script. It'll tell you if login worked or not.

imap_login_test.py

#!/usr/bin/python3

import imaplib

# If you need to use SSL/TLS encrypted secure connections,
# then set this to True
SSL_ENABLED = False

# Change this to be your IMAP server hostname.
IMAP_SERVER = 'hostname'

# Change this if you listen on a nonstandard-port.
# 143 (non-SSL/TLS) and 993 (SSL/TLS) are standard ports.
# Don't change if you use those ports.
IMAP_PORT = None

# Change these accordingly.
USERNAME = 'username'
PASSWORD = 'password'


if __name__ == "__main__":
    try:
        print("Connecting to mail server...")
        if SSL_ENABLED:
            conn = imaplib.IMAP4_SSL(IMAP_SERVER, IMAP_PORT if IMAP_PORT else 993)
        else:
            conn = imaplib.IMAP4(IMAP_SERVER, IMAP_PORT if IMAP_PORT else 143)

        print("Connected to server.\n")
    except:
        print("Could not connect to mail server.\n")
        exit(1)

    try:
        print("Attempting to login...")
        data = conn.login(USERNAME, PASSWORD)
        if data[0].lower() == 'ok':
            print("Logged in successfully.\n")
        else:
            raise Exception("Login failure.\n")
    except:
        print("Could not login to mail server.  Make sure you are using the proper credentials.\n")
        exit(2)

    print("Connected and logged in without issue.")
    exit()