Fastmail is the closest thing to Folio.
Here's where they actually differ.

It would be easy — and wrong — to draw Fastmail as a service that scatters your identities into separate mailboxes. It doesn't. Fastmail puts many custom domains and aliases into one unified inbox on a single account (it supports on the order of a hundred domains), and when you reply, it picks the From address the message was sent to. That is the same multi-identity behaviour Folio markets. If you've seen Folio described as uniquely "one inbox, right From every time," know that Fastmail does that too.

So the real question isn't whether your identities live in one inbox — on both, they do. It's how each product helps you stay oriented inside that one inbox, and how it prices the privilege of running a portfolio of domains.

Folio's genuine differences are narrow and worth stating plainly. First, per-domain colour: every bound domain gets a deterministic colour stripe that follows it through the inbox list, the thread view, and the composer, so a portfolio reads as a portfolio at a glance — Fastmail organises by label and search rather than by colour. Second, DKIM with no manual step: Folio generates a per-domain key automatically the moment you bind a domain, where Fastmail walks you through a domain wizard. Third, framing and price: Folio is explicitly a single-operator, multi-domain product with flat domain-led tiers — $2.99/mo for up to 3 domains, $12 for up to 10, $29 for unlimited — and no second-user assumption.

None of these is a knockout. Fastmail's Individual plan is $5/mo and isn't billed per domain either. If per-domain colour and automatic DKIM-at-bind match how you think about a portfolio, Folio fits; if they don't, Fastmail's maturity is the stronger draw.

Fastmail's Calendar and Contacts are first-class, with CalDAV / CardDAV and an in-app event editor. If you depend on those — especially on iOS — that is a real, decisive Fastmail advantage. Folio ships a focused agenda inside the inbox: invites land as readable cards with RSVP buttons (accept, decline, propose new time), recurring events expand from RRULE, conflicts flag at accept time, and your agenda is exposed as a subscribable iCal feed any calendar app can read. There is no in-product editor for organising your own meetings yet, and Contacts / CardDAV remain out of scope.

On craft generally, Fastmail has two decades behind it: faster keyboard shortcuts, better search, a more polished iOS app, and JMAP. Folio is younger and more opinionated; its craft is invested in the editorial reading surface — typography, the colour stripe, the live UI mocks in the manual. Whether per-domain colour and flat portfolio pricing outweigh Fastmail's maturity is a matter of temperament, not a factual win for either side.