If you see the following error message in your email client when attempting to check your emails, this means you are attempting to use an unencrypted connection:
Plaintext authentication disallowed on non-secure (SSL/TLS) connections
For security reasons only SSL or TLS connections are accepted so you will need to configure your email client accordingly. The instructions differ depending upon your particular email client, you should look within the account settings section for the option to establish secure connections for both the incoming and outgoing mail servers.
Windows Mail Settings