I made the attempt this weekend to switch to FastMail from Google for my custom domain. It lasted less than an hour. Here's why.
By way of (brief) introduction, FastMail is a direct competitor to Google GSuite. It is great for folks who are privacy concious. Their plans are affordable, and their UI is slick. Included is an importer tool that will allow you to move all of your email from any IMAP-capable account (which includes Google) into your new FastMail account.
I came incredibly close to making the migration. I had MX records on my primary domain switched over, did some initial tests, and was beginning to switch over some of my secondary domains before I was stopped by an incoming email. This was an email that I wanted to save for later, but alas: I could not. Unlike Google Inbox and the "new" Gmail, I cannot snooze emails in FastMail and have them reappear in my inbox later on. My only options were to leave the email in my inbox or to send it to the archive. This single feature was my dealbreaker.
A few months back I attempted to switch away from Google's interface and toward a desktop email client. I tried to use Airmail and Apple Mail. I realized pretty quickly that snoozing emails and having them reliably reappear in my inbox was an essential workflow for me. Without this work flow a lot of things get missed when I inevitably declare inbox bankrupcy. Both Apple Mail and Airmail offer or can be extended to offer that feature, but they didn't work effectively unless the application was actively running on my machine. Given the frequency with which I would snooze something on my computer and need to see it on my phone later on, I couldn't tolerate that limitation.
Therefore, for the moment, I'm locked into the Google life. However, the limited time I spent tinkering with FastMail looked interesting. There are other apps that provide the functionality I'm looking for (e.g. Newton), but the idea of a service that proxies my email through their servers makes me a bit leery... it doesn't feel like that should be required to get the effect I'm looking for. Also, if I have to go through their servers to get an experience that makes sense, have I really gained much by not being in the Google ecosystem? (Especially given I'll be paying an annual fee for the pleasure?)