Comparable email service? Not sure on what terms to compare. But take a look at the below privacy focused (or privacy respecting) services before you look at Fastmail (which can be quite expensive if you need multiple mailboxes):

Posteo.de (no custom domain support), runbox.com, mailbox.org, mailfence.com, migadu.com, mxroute.com (if you don't mind hosting in the U.S.), ProtonMail (needs a bridge software to use IMAP) and Tutanota (no IMAP support)

If you'd like to support the next generation JMap (which Fastmail develops), then getting a paid subscription on Fastmail could help.