Once you've used a service like Intercom, FrontApp or even Google's "Inbox" you realise emailing can be far more productive. It's a shame that companies like FastMail gave up on trying to improve the experience.
On top of that, it sounds like you really want something other than email.
And there's no good standards compliant way to implement that, so it'd mess up third party clients. This is why Gmail is such a broken experience if you aren't using Gmail's own proprietary Google-blessed apps.
* Create a folder 'On Hold'
* Move emails there
* Check in a couple days.
If you want alternatives, check AirMail. I find it's a good client on iOS and OS X that implements what you want
With Google Inbox (inbox.google.com) I can Snooze an email for [x] number of days and it'll be gone from my inbox and I don't have to think about it until it comes back. I use Inbox for personal email but I'd rather not be.
Airmail doesn't have a web client, and that's where most of my emailing takes place, so unfortunately it's not an option.
We use FrontApp on top of FastMail for customer service, it provides a bunch of useful features on top of email, such as the one I just mentioned. I could use it for my personal email but that would mean paying two monthly subscriptions. Plus it doesn't have spam filtering built in.
And... You've just lost standards compliance so you can't use any standards compliant clients. This is a terrible hack of a workaround and you know it :)
I use FastMail for my work email and it's ok, but I wish these guys would introduce a few new features from time to time. Why in 2018 do I not have the ability to "Snooze" an email and come back to it at a later date? or add notes to an email that don't get sent to the person I'm conversing with? or consolidate emails from multiple conversations into a single conversation? Yeah FastMail your service is a better alternative to Gmail because I don't have to worry about being locked out of my email one day for no apparent reason, but why aren't you innovating? Email with Fastmail is just as tedious and boring as it was 10 years ago.
tl;dr - less politicised blog posts, more making your service not suck, please.