What is iCap Inbox Assistant?
iCap Inbox Assistant is a Thunderbird junk mail add-on that finds real mail stuck in your Junk folder and flags phishing that gets through to your Inbox. Every message gets a verdict and a plain-language list of reasons, so you can see why it looks real or risky instead of trusting a hidden score.
The free version runs entirely on your computer. It makes no network calls, needs no account, and never sends your mail anywhere.
Status: in testing now, coming soon to Thunderbird Add-ons. Requires Thunderbird 140 or later.
What problem this Thunderbird add-on solves
Spam filters make two kinds of mistakes. They send real mail to Junk, such as booking reminders, receipts and account notices, and you only find out when something is missed. They also let some phishing through, because a well-made fake can pass the same tests a real message does.
Thunderbird’s junk filter and your mail provider’s filter both decide quietly. iCap Inbox Assistant is a second reviewer that sits on top of them. It doesn’t replace either one; it explains what they found and checks what they don’t.
How the free version works
- Open Junk review. Click the iCap Inbox Assistant button in Thunderbird’s toolbar, the Junk review icon in the left-hand app bar, or right-click a Junk or Spam folder and choose Review junk. It reads today’s Junk (or the last 3, 7 or 30 days) across all your accounts, or just the folder you picked.
- Each message is checked on your computer. The add-on reads the message’s headers, text and attachment names. It never opens links, never runs attachments, and never sends anything out.
- Messages are sorted into three groups: Might be real, No opinion, and Suspicious or phishing. Each one lists the reasons behind its verdict. Show all messages, or only unread or read ones.
- You decide. Not junk moves a message to your Inbox. Always trust a sender moves its mail out of Junk and treats that sender as trusted from then on. Each group also has Mark all read and Mark all as junk, with a count of the messages they’ll act on. Nothing is moved or deleted unless you click.
Junk review isn’t the only place it works:
- A count on the toolbar button. While a Junk folder is open, the button shows how many of today’s messages there might be real.
- Right-click any message for Check with iCap Inbox Assistant, Not junk, or Not junk and always trust sender.
- A check on every message you open. The Check this message button shows the same verdict and reasons. Messages that look like phishing get a red ! badge, and suspicious ones a yellow ?, so a risky message in your Inbox stands out.
What the free checks look at
The checks are fixed rules built on published mail-filtering best practice, the same signals mail administrators use. They work the same for anyone’s mailbox and any mail provider.
- Sender authentication. Whether the message passed SPF, DKIM and DMARC, read from the results your mail provider recorded, including mail forwarded through Gmail.
- Your mail provider’s spam verdict. The add-on reads the spam-filter results many providers write into each message, including Microsoft 365 and Outlook.com, iCloud Mail, and servers running Rspamd or SpamAssassin. It learns which results your provider writes, so a forged copy added by a sender is ignored.
- Who it says it’s from. Look-alike domains (paypa1.com), misspelled brand names, a well-known brand’s name on mail from somewhere else, a fake address shown as the sender’s name, hidden characters, and senders posing as your IT department or mail administrator.
- Links. A link that shows one address but goes to another, links to bare internet addresses, and sign-in pages hosted on free web hosting services. Links are compared, never visited.
- Attachments. Programs and scripts, files hiding their real type behind a second extension, disk images, attached web pages and macro-enabled Office files. Only names and types are read; attachments are never opened.
- Callback scams. “If you didn’t make this purchase, call us” messages that push you to a phone number instead of a link.
- Your own history. Mail from senders you’ve read and kept before counts in their favor.
- Who marked it as junk. Whether your mail provider flagged the message, or it was marked as junk in Thunderbird itself.
In testing on public phishing collections, the free checks flagged about 62% of phishing messages, while flagging 0.5% of legitimate mail. Most of what they miss is plain-text lures with no telltale sender, link or attachment, which is what the Pro AI review is for.
Privacy: your mail stays on your computer
- The free version makes no network calls. Your mail, your rules and your settings stay in your Thunderbird profile.
- The add-on never deletes mail and never acts on its own. Every move is a button you click.
- It works with any account Thunderbird holds: IMAP, Microsoft and Outlook.com, iCloud, Yahoo, Gmail and local folders.
Pro: AI review (coming later)
Pro is in planning and isn’t available yet. It adds AI review for the messages fixed rules can’t settle, using iCap AI credits, the same credits model as our iCap SEO WordPress plugin. You won’t need an AI account or an API key. Planned features:
- “Is this legit?” An AI verdict and explanation for one message, for the plain-text lures and unusual requests that rules miss.
- AI Junk sweep. AI reviews the Junk messages the free checks have no opinion on, and lists the ones likely to be real.
- Thread summary. A short summary of a long conversation.
- Draft a reply. Saved as a draft for you to edit. Never sent automatically.
- Suggest a folder. Where a message probably belongs.
How AI review will handle your mail:
- Off until you turn it on. AI features need your explicit opt-in before any message content leaves your computer.
- Only what you ask about. Only the message you choose is sent, as its headers and a trimmed excerpt, to the iCap service. Nothing is sent in the background.
- Everything free stays free. Junk review, the checks, badges and trust rules never need Pro, and there are no upgrade pop-ups.
Pricing will be announced when Pro launches. This page will be updated as features ship.
Frequently asked questions
Does it replace Thunderbird’s junk filter?
No. Thunderbird’s filter and your mail provider’s filter keep working as before. iCap Inbox Assistant reviews what they decided and explains it.
Does it work with Gmail, Outlook.com and iCloud?
Yes. It works with any account in Thunderbird. Providers that write their spam-filter results into each message, like Microsoft and iCloud, give it more to explain.
Does it see my mail server’s Spam folder?
Yes. Besides Thunderbird’s own Junk folder, it reviews folders your mail server files spam into, such as Spam or Bulk Mail.
If I trust someone on Gmail, does that trust all of Gmail?
No. Trusting a sender on a free email service like Gmail, Outlook.com or Yahoo trusts only that address. Trusting a company sender trusts its whole domain.
Will it move or delete mail on its own?
No. It only moves or marks messages when you click a button such as Not junk, Always trust or Mark all as junk, and it never deletes anything.
Does the free version send my mail anywhere?
No. The free version runs entirely on your computer and makes no network calls.