Didn't work for me

Uhm, but are you talking about one of the theme of the plugin? All of them?

I’m talking about your newsletter plugin! I spent a lot of time to make it work, followed your instructions (thank you!) but in the end, nothing really worked: Images didn’t show up, links worked only extremely slowly, etc. I had used your default theme as you recommended but also had tried others before.
In my review I mentioned that I’m using the wordpress theme pinboard. But this has probably no influence on the outcome, I assume.

Exactly the theme has nothing to do. Slow link can be caused by slowness of the blog, since they are tracked, but you can disable the tracking. About images that don’t show up, I don’t know, if you see it on the composer they should show everywhere. Probably there are something specific in your installation since the plugin just create an HTML page sent by email.

Of course if you create an email with mailchimp which is sent from their certified servers, the email client, specially the online email readers, can act very differently when displaying it compared to displaying and email from a mail server of a web hosting company.

Anyway, only with a list of detailed issue it’s possible to determine if there is a solution and if the work worth to solve it.

images not opening is down to you email software like outlook etc they all have different settings for your security.