Zoho Mail, Fastmail and HEY are not on most senders' radar. Their combined consumer market share is a rounding error. Their share of the inboxes you actually want to reach is much larger — founders, engineers, product people, and the privacy-aware professionals who moved off Gmail on purpose. Treat them as three distinct filters with three distinct failure modes.
GlockApps does not seed Zoho, Fastmail or HEY. Mailgenius does not. Most ESPs lump them into "other" on their dashboards. You cannot measure what you cannot seed, and almost nobody seeds these three.
Who uses each
- Zoho Mail — heavily tilted toward small and medium businesses, especially in India, the Middle East, and the LATAM tech ecosystem. Many Zoho One customers take the mail as part of the bundle. A lot of B2B mail lives here.
- Fastmail — independent Australian provider. Paid only. Users tend to be longtime IT, sysadmin and engineering professionals — and a growing cohort of ex-Gmail privacy switchers.
- HEY — Basecamp's mail product. Paid only. Users are founders, solopreneurs, early-stage operators. Small but highly-influential audience.
If you are cold-emailing a B2B list of tech workers, founders or decision-makers, you will hit addresses on all three. If you have no read on how those three filter, you have a blind spot proportional to your ICP density.
Zoho Mail: SpamAssassin plus custom rules
Zoho runs a filter stack built around SpamAssassin with Zoho custom rules layered on top. The custom rules are business-focused: sign-in alerts, invoice patterns, sales-outreach fingerprints.
Two Zoho-specific quirks worth knowing:
- Strict handling of sender display name / from mismatch. If your display name is "Jane @ Acme" and your From domain is
mailings.acme.com, Zoho is more likely to spam it than Gmail is. Keep the visible From domain tight. - Heavy weighting on Received chain. Zoho inspects the hop-by-hop headers and penalises obvious third-party ESP chains that do not match the From domain. Custom return-path and aligned DKIM help visibly.
Authentication
SPF, DKIM and DMARC are mandatory. Zoho is particularly strict on DMARC alignment — relaxed alignment is tolerated but strict is scored better. p=none with no reporting configured is treated as a weak signal.
Fastmail: SpamAssassin + Bayesian, hates image-only
Fastmail runs SpamAssassin with a heavily tuned Bayesian layer trained on Fastmail's own user-reported spam. The filter is famously good at two things Gmail is average at:
- Detecting image-only marketing HTML. A message that is 90% images with a hero graphic and a single CTA button will almost certainly be filtered at Fastmail, even if authentication is clean.
- Detecting marketing-template patterns. Wide tables, multiple CTA banners, common newsletter footers, and similar structural fingerprints get penalised.
Fastmail users are also more likely than average to configure custom sieve rules — the filter can be made arbitrarily strict by the user. A small percentage of Fastmail mailboxes are impossible to reach via any automated outreach, by design.
HEY: the Screener
HEY is the outlier. It does not have a traditional spam folder. It has a Screener: every sender you have not previously approved lands in a holding area where the user explicitly decides, per sender, whether to let them through. If the user does not approve, every subsequent message from that sender silently goes nowhere.
The implications for cold outreach are severe:
- You cannot optimise your way into a HEY inbox with authentication or content. A HEY recipient either approves you or they do not.
- First-touch messaging matters disproportionately. The Screener shows the user your From, subject and a preview. That preview decides whether the relationship ever starts.
- HEY also maintains more aggressive blocks on known cold-outreach sending infrastructure than most providers. Certain ESPs are effectively pre-blocked.
For HEY specifically, the question is not "did it inbox?" but "did the recipient approve it on the Screener?". A placement test tells you one and not the other — but knowing the Screener verdict ("shown" vs "blocked outright") is still materially useful data and is the closest signal available.
What breaks in each
- Zoho: mismatched From / return-path domains, missing DKIM, aggressive marketing subject lines.
- Fastmail: image-heavy HTML, multi-CTA newsletter layouts, user-trained Bayesian rules specific to that mailbox.
- HEY: everything, unless the recipient approved you on the Screener. Plus elevated blocks on known cold-outreach ESP infrastructure.
Shared authentication expectations
All three expect the full authentication stack: SPF with alignment, DKIM signing on the From domain, DMARC at least p=none with reporting and ideally p=quarantine or stronger. If authentication is shaky you will fail at all three simultaneously before any filter-specific quirk even matters.
Run a SPF/DKIM/DMARC check before thinking about anything else on this page.
One free test that covers all three
Our placement tool seeds Zoho Mail, Fastmail and HEY alongside Gmail, Outlook, Yahoo, iCloud, ProtonMail, GMX, T-Online, Orange, LaPoste, Free.fr, WP.pl, Onet.pl, Mail.ru, Yandex and more — 20+ providers in a single test. Send one message to a generated address. Get Inbox / Spam / Screener / Missing per provider with raw headers and authentication results. Free, unlimited, no signup.
It is the only way to get a single-shot read on these three providers at once.