Registration with MailChimp double opt-in
Ultimate Member`s settings
Ultimate Member has two places where you may configure "double opt-in":
1. Option "Enable double opt-in" in the settings allow to enable "Opt-in" behavior for all audiences.
[Ultimate Member > Settings > Extensions > MailChimp]
2. Option "Enable double opt-in" in the audience allow to enable "Opt-in" behavior for the certain audience. This option has higher priority, option in the settings have sense only if option in audience is "Default".
[Ultimate Member > MailChimp > Edit audience]
Look to the "Registration Status" option in the "Registration Options" metabox on the role settings page. New user will be added to the MC audience after approvement. New user would not be added to the MC audience immediately if you set option "Registration Status" to the "Require Email Activation" or "Require Admin Review". But "double opt-in" behavior doesn't depend on "Registration Status" option.
[Ultimate Member > User Roles > Edit Role]
MailChimp has it own "Enable double opt-in" option for the audience, don't forget to enable it. You also should set "Default From name", "Default email address" and other options.
[MailChimp > Audience > Settings > Audience name and defaults]