If you see either of the following error messages 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
[Errno 10054] An existing connection was forcibly closed by the remote host.
For security reasons only 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.
Accessed via File > Account Settings > Server Settings:
Incoming mail settings
Outgoing mail settings
Windows Mail Settings