Errors print error: <message> on stderr and exit 1.
SymptomLikely causeWhat to run
no accounts authorizedNothing stored yetsetup.md
no OAuth client storedGmail used before set-clientmailkit auth set-client <client.json>
no token for …Account missing or half-registeredmailkit auth add … for that provider
invalid_grant / doctor FAIL on GmailRefresh token revoked or expiredmailkit auth add you@gmail.com again. Publish the OAuth consent screen to Production.
Fastmail doctor FAILToken revoked or mistypedMint a new Mail token; auth add --provider fastmail
Bridge is not reachable / Proton doctor FAILBridge not running, or wrong portOpen Proton Mail Bridge, sign in, retry
Bridge password rejectedNot the mailbox passwordCopy the Bridge mailbox password, not the Proton login
Bridge certificate changedBridge cert rotatedmailkit auth add --provider protonmail you@proton.me again
'send' is not supported on fastmailWrong accountUse a Gmail or Proton Mail -a
'thread' is not supported on protonmailVerb is Gmail-onlyUse read / show
UnsupportedQuerySearch syntax this provider does not compileSee read.md
refusing to send without --yesMissing flagGet a human yes, then add --yes
--attach can't be combined with --draftFlag mixAttach when drafting, or send a new message
403 accessNotConfiguredGmail API not enabledEnable Gmail API on the Google Cloud project
Google “app not verified”Personal Desktop clientAdvanced → continue. Expected.
Keychain / “no accounts” in a sandboxAgent sandbox blocked KeychainRun mailkit unsandboxed

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Doctor first

mailkit auth list
mailkit auth doctor --json
The provider field tells you which mailbox you are actually talking to.

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Proton Mail ids

If show says the id is not a Proton Mail message id, you passed a Gmail hex id to a Proton account (or the reverse). Use ids from read --json on that account.