It fills the gap between large commercial softwares for large companies, and for small companies. Most email hosts are just basic until you start bringing in video calls, chat, etc. Missive has that and more, and I think is the #1 email choice for small companies.
I like that it's free, I like that it's open-source, and the fact that it allows IMAP lets me have this on all 3 laptops and it's synced. I would never consider using it as my primary email client, however; just too clunky and can't navigate around fast enough the way I do with Gmail. And not even close to having all the features I'd need that Gmail does: 30 second undo-sending, canned responses, easy-to-create signatures for each email address (I literally toggle over to Gmail, compose a message, and paste my sig into the body of Thunderbird emails, so clunky), all spam ends up in my inbox even after I've marked enough as spam that it should know, email labeling and filtering... Gmail is just way more advanced and feature-ful. But, Thunderbird does beat my ridiculous RoundCube webmail interface!
Thunderbird is a software very simple to use, but efficient in every aspect. We don't have a specific need, apart to manage email in both directions, and Thunderbird does it's job in the best way. I would like to see a more friendly interface, with some colours more, to help the sight in the email searching. When we have many of them, sometimes it's difficult to find the one that we need.
I can only compare to free services or to Outlook. I cannot go back to Outlook... Truly. Outlook hits all the right marks for a business, and Missive exceeds those marks and sets new standards. I joined a company that had already chosen Missive. I not only was able to pick it up intuitively, but I felt proficient at it within a couple of weeks and already felt my workflow and communication skyrocket.
Overall efficiency in our communication has probably tripled. We can draw a collaborative conclusion, in-office or remote, desktop or remote, within minutes assuming we are all available.
We realize we are still "under-utilizing" this app. It can be a powerhouse if mastered.