First of all, to send emails you'll need an API key, from Sendinblue. Here is how you get it:
Log into your Sendinblue account and go to SMTP & API from the top-right menu:
Once you're on this listing, you can either copy an existing API key or create a new one by click on the generate button and set a name for it:
Now you can simply copy the API key into the AcyMailing configuration.
You'll need one more thing to send email properly from AcyMailing: set the sender information. To do that you'll have to create a new sender into your Sendinblue account. Go to the menu Sender & IP on the top-right menu:
If you don't already have a Sender set you'll have to create a new one by clicking on add a new sender, set the name, email and save it:
Once you've set these information you'll have to set them into the AcyMailing configuration under the tab mail settings: