POP3/IMAP synchronization is disabled by default. If you want to use third-party or mobile apps to sync your email, you need to enable this feature in your settings.
Step-1 In the Email tab, click Settings.
Step-2 Click POP3 & IMAP.
Step-3 Activate the checkbox that says Enable access to this account via POP3 and IMAP.
Step-4 Click Save.
Watch Tutorial to ENABLE IMAP Access in GMX Account