If you use internal authentication of users to the Zimbra directory, you can create password rules that require users to create more secure passwords. You can set a combination of password length, password uniqueness rules, and password lifetime. Password rules can be configured in COS or individual accounts.
See Password Options when Authentication is Internal for a description of the rules you can set.
In addition to setting password rules, you can configure account lockout rules for when invalid passwords are tried.
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