MailRules vs Thunderbird message filters
Thunderbird's filters are built into the app and run when Thunderbird fetches new mail. They match properties such as Subject or From. MailRules runs without a desktop app, on the server side of IMAP, and follows rules written as sentences.
| MailRules | Thunderbird message filters | |
|---|---|---|
| When it runs | All the time, as a small server process. It watches your mailbox with IMAP IDLE and files new mail seconds after it lands. | When Thunderbird gets new mail, after sending, when archiving, or when you run filters by hand. |
| Which mail it sorts | New mail in the inbox. Cleanup sorts mail already in a folder you choose. | Automatic filters apply to the inbox only. Run by hand, a filter can be applied to any folder. |
| How rules are written | Type or dictate a sentence such as "receipts, but not shipping updates". Add exact conditions on sender, domain, subject, any header or attachments when you want them. | Pick a property, a test and a value for each condition, such as Subject contains "discount", and match any or all of them. |
| What a rule can do | Move, archive, trash, mark as junk, flag, mark read or unread, or keep. | Move, delete, forward, star, reply and more. |
| Other devices | It moves mail on the server, so your phone, webmail and Thunderbird all see the same folders. | Filters live in Thunderbird. Each account has its own set. |
| When it's unsure | Below your confidence threshold, the email stays in your inbox and waits in Needs review. Every action can be undone for 30 days. | A filter matches its conditions or it doesn't. |
| Price and licence | Self-hosting is free under AGPL-3.0; you pay your model provider. Hosted cloud is coming: Personal $5 a month, Pro $10 a month, Team $8 per seat. | Free and open source, as part of Thunderbird. |
A filter that lives in one app
Thunderbird’s own help says filters run when you receive new mail, after sending, when archiving, or when you run them by hand. Automatic filters only look at the inbox. They run inside Thunderbird, so mail you read on your phone first may not have been sorted yet. MailRules runs as a separate process and moves mail on the server, so every app sees the same sorted folders.
Properties and tests versus a sentence
A Thunderbird filter is a set of property, test and value rows: From is [email protected], Subject contains “discount”. That’s precise, and it’s the right tool for a known sender. It can’t express “newsletters I never open” or “recruiters I haven’t replied to”. In MailRules you write that sentence. It checks senders you’ve corrected and exact conditions first, then asks a small model, and a larger one only when that model is unsure.
Seeing what happened
Each MailRules action records the rule, a confidence and a one-line reason. If one is wrong, undo it and correct it in one click, and MailRules can remember that sender. Trash goes to a MailRules Trash folder first, so nothing is permanently deleted.
When Thunderbird message filters is the better pick
Stay with Thunderbird filters if Thunderbird is the app you read mail in and your computer is usually on. They're free, they need no server and no AI model, and they can forward and reply, which MailRules can't. They also handle outgoing and archived mail, and you can run them on any folder whenever you like.
Questions
Do Thunderbird filters work when Thunderbird is closed?
Thunderbird applies its filters when it gets new mail, so they act when Thunderbird is running and checking that account. MailRules runs on its own, so mail is sorted before you open any app.
Can I keep using Thunderbird with MailRules?
Yes. MailRules moves mail on the IMAP server, and Thunderbird shows the result like any other change to your folders.
Should I turn my Thunderbird filters off?
Two systems moving the same mail is hard to follow. MailRules starts in dry-run mode, so you can see what it would do before you decide which filters to keep.
Does MailRules work with POP accounts?
No. It needs IMAP, because it moves mail into folders on the server.
Can MailRules forward or reply like a Thunderbird filter?
No. MailRules only files mail and never sends anything.