Emacs

I've been a Fastmail user for several years now. I use the masked mail feature extensively. I love controlling my own email domains. Support is 10/10. Very reliable service.

People use Emacs for mail? The more you know I guess...

Very happy with Fastmail and mbsync (and notmuch+gnus, but mu4e is probably great too). I've never tried sending from a masked email address, I just use those for e.g. shopping accounts. But it's easy to send from an alias, just change the From: header :)

Wow, Google actually will take down IMAP. I wouldn't have taken your concern seriously, but I looked it up.

Changes to Gmail syncing with other email clients

To keep your account more secure, Gmail no longer supports third-party apps or devices that require you to share your Google username and password. Sharing your account credentials with third parties makes it easier for hackers to gain access to your account.

Instead, look for the option to 'Sign in with Google', which is a safer way to sync your emails to other apps. Learn about Sign in with Google.

Gmail users with a personal Google Account

In the coming weeks, the option to 'Enable IMAP' or 'Disable IMAP' will no longer be available. IMAP access is always enabled in Gmail, and your current connections to other email clients aren't affected. You don't need to take any action.

https://support.google.com/mail/answer/7126229?visit_id=638336166820267774-2369444182&hl=en-GB&rd=1

Late, but yet another Fastmail user, with Gnus.
Like the other comment says, the feature you are looking at is Aliases, I have my primary address and then subscriptions@mydomain.com, work@mydomain.com, etc with rules that send them to different directories.

I wrote a package to sync Fastmail's CalDAV calendars to Emacs' built in calendar, and now I can user FM good apps on the phone and all their services in Emacs when on the computer.