After a short trial with Spark I’m back to Apple Mail. I experimented with Spark for a month because I like the feature set but I always run into a few quirks that are frustrating. For example, I recently tried to paste text from a webpage into Spark only to get part of it as HTML code. When I do the same in Apple Mail it pastes the text properly.
I read several posts on Mailbutler. My questions are:
I found it to be reliable, and really liked the features. I ended up phasing out my use of it on my iMac, because it really is mouse-driven (or at least it was, before I stopped using it. Can’t speak for it now). My main use is to defer emails, and I find I can do it much quicker with a combination of MsgFiler and Sanebox, using keyboard shortcuts.
I still have Mailbutler on my MacBook Pro, for the “Send Later” feature. On my iMac, I use another tool that requires my Mac to be powered on and awake, but my MacBook isn’t powered on 24/7.
If you like using the mouse, or if it has changed to better support keyboard shortcuts, it is worth a look.
EDIT: Here was my review from November 2018. My recollection is it got more reliable over time, but then I eventually stopped using it because of my desire for keyboard shortcuts.