The most common cause for this error is that the email server does not support the establishment of less secure connections. It is primarily due to the security implications that may arise when using insecure configurations; therefore, by default, most email service providers, including Google, disable these settings. However, since some clients use these settings to send emails, the absence of these configurations forces the client to fail.
By default, Google restricts the use of less secure applications establishing connections to secure the email account. Previously the user could change these configurations to enable less secure applications to connect. However, recent advisories from Google show that this feature will no longer be available for users, and all accounts with this setting enabled will not continue with the same configurations.
The Google advisory explains further information regarding this change.
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