This is, of course, a great deal. Email platforms are expensive over time, so I'm willing to forgive some rough edges to save money. But Acumbamail is a solid step below platforms like ConvertKit, MailerLite, and even Mailchimp.
All the features are here, nominally. But not as good as the competition. For example, there are some, but not as many, segmenting capabilities as the other platforms.
You can maintain separate Lists, but that gets unwieldy over time compared to a Tag-based system.
There are Automations, but they are bare-bones, the Automation builder functions like a Beta product, and lacks such basic features as showing the title of each email in the builder view. Likewise, for every. single. email. in the builder, you need to manually set parameters such as subdomain, when they should be global defaults.
Not all the list import options function equivalently — options such as "past in" lack list update features that are available in the CSV upload workflow. Their wordpress plugin is essentially abandoned. I could keep going but you can see that the whole thing feels like a beta test.

It DOES deliver emails, and with your SPF, DKIM and DMARC set correctly, the deliverability is good. It has some integrations, and I was able to set up an automation from a wordpress site with Pabbly. I am happy with the email testing feature, which is a nice add-on. I am hopeful that with the influx of cash from Appsumo sales, they will be able to improve and polish the many rough features. I do wish them success.