This plugin is a mess. Although it is promising - you basically get the Mailjet UI in a dedicated WordPress-page - the lack of thought the developers have put into this is astounding. It is perhaps fine, if you deal with 1 single list and you are using 1 single subscription-box on your site, but the moment it gets a bit complicated, the plugin is messing around so far as it became unusable for me.
How did that happen?
If you create a widget, you select a contact-list for the widget to use. If you save your settings, this widget gets an ID. If you do not have a widget area (as it is more and more common with Gutenberg), you can still use the plugin by using the shortcode it provides. So, say, you create a widget for your general marketing list (ID = 1), the shortcode is something like [mailjet_subscribe widget_id="1"]. So far so good. You take that, you use the shortcode-block and there you have a nice subscription widget right inside a Gutenberg-block.
Then, later, you add another marketing-list (in Mailjet) for a special occasion, say a black-friday-promotion. You proceed as before: You create a new widget (ID = 2), select the new contact-list and use the newly generated shortcode ([mailjet_subscribe widget_id="2"]) on your site. All looks good and makes sense.
And suddenly, the marketing department calls: ALL subscriptions go into the black-friday-promotion-list, no matter which form you fill in. Not a single contact gets saved into the general list any more. Looks like, no matter the ID in the shortcode, the Mailjet plugin uses the highest ID it finds. This is especially tricky, if you create another widget AND delete the old one. Then, at the same time, another person does the same, realizes that it has been fixed already, and now you have a widget deleted that had a higher ID than the one you just created. This is makes the situation even weirder: You have just created a widget for the general list and everything goes into the promotion-list.
Noone seems to have thought this situation through. Not a single person. This is such a massive fail for a company specializing in providing marketing and transactional email-sending services, that I can only call this plugin unready for production and tell everyone to NOT use it currently.
Mailjet ≠ Mailjet-plugin
To be fair, we still use Mailjet, the service, but each and every subscription widget is generated with their own subscription-widget-generator (and not this *+!#-plugin), which produces an iframe (and a script-link) that you can use on your page. This works nicely, although editing the iframe is a one-way-street (once you activate HTML-mode, there is no going back) and the basic layout is a catastrophe. But it at least works and subscriptions go to the right lists.
So, all in all: Mailjet as a service is a nice tool, the Mailjet-plugin for WordPress is a mess and I absolutely do not recommend using it for newsletter-subscriptions. (I have not tested the transactional mail functionality.)
The version I have used is: 5.2.4.