There are different factors that can cause such an error. So please share with us the configuration for your e-mail port.
Also let me know whether your IMAP e-mail port is configured to receive requests from an e-mail server that uses NTLM for authentication?
One more thing is to check mail.jar file which is located in <IS_DIR>/lib/jars directory.
You can also go to Settings > Extended > Show and Hide Keys and add watt.config.systemProperties and set the value to true. This helps you debug IMAP port issues on server.
Try finding it by opening server.bat or server.sh located under <IS_DIR>/bin (directory depends on your wM version) and then search for JAVA_OPTS line and check if you can find the property β-Dmail.imap.auth.ntlm.disable=β and check if itβs true or false.