hello Frédéric AZOULAI,

Sorry for late reply to this,

You can just configure Joomla to use the SMTP details on Mandrill. This would be activated throughout your Joomla site.

In other words, if any scripts or extensions that tries to send out emails, it would route via Mandrill too.

You just need to set this up on Mandrill and obtain the SMTP details. Then configure this in your Joomla Global Configuration area.

Regarding the mailChimp API key, you have to setup in your backend > Easyblog > setting > workflow > Subscription > Mailchimp Integrations

Hope this help.