Postbox is gone. eM Client bought it in October 2024, sales and development stopped that month, and user support ended on 22 December 2024. If you liked its search, its attachment browser and its threaded conversations, the closest options today are eM Client, which took on Postbox’s users, and Mozilla Thunderbird, which Postbox was built on and which now ships monthly releases. For email marketers, the bigger change since 2009 is that the mail app on your desk matters less than authentication and privacy. Gmail and Yahoo have required SPF, DKIM, DMARC, one-click unsubscribe and low spam rates from bulk senders since 2024, and Apple Mail Privacy Protection has inflated open rates since 2021.
A first look at Postbox in 2009
The review below dates from mid-2009. Postbox had launched a public beta for Mac OS X and Windows in February that year, and the app picked up early fans fast. One of them was Dean Proctor, a director of information systems, who praised it all over Twitter. Many people tried it for the same reason: the standard mail apps of the time, Apple Mail included, felt stale, and few desktop clients had anything new to offer.
The early complaints
The 2009 beta had real rough edges. They are worth recording because they show what a young desktop client was up against back then.
- No sound for new mail. Even with the preference set to play a sound when mail arrived, the beta stayed silent.
- Badges only for the inbox. The red count on the Dock icon only appeared when a message landed in the main inbox. Mail that filters sent to subfolders triggered no alert at all.
- Slower server checks. Postbox seemed to check for new mail less often than Apple Mail.
- A failed import and crashes. Importing from Apple Mail failed once, and the app crashed three times. This looked like a one-off, because other early users on the same team ran it without problems.
Apart from those annoyances, early users saw Postbox as a standout app.
Search was the standout feature
Search was the main reason people switched to Postbox. Besides names and subject lines, you could search for a specific attachment. A menu with Attachments, Images, Links and Contacts tabs opened a new tab listing every email that matched that type. Date searches in plain words worked too, such as “before July 2009”.
An early user named Patrick summed up why the search stood out:
Postbox is awesome because it harnesses the major power of Gmail: search. By indexing your email and using some very flexible searching algorithms it returns more results than any other email client I’ve used.
That comparison to Gmail fits the period. In 2009 Gmail had made “search, don’t sort” normal in webmail, and Postbox was one of the first desktop clients to bring the same idea to local mail.
Message threading
Threading kept every related message in one conversation, sorted by date. Early users found it so obvious they wondered why nobody had done it sooner.
Threading is now standard. Gmail, Apple Mail, Outlook, Thunderbird and eM Client all group conversations, so this is no longer what sets a client apart.
Social media, topics and composing
These were the smaller features of the 2009 build. Two of them are clearly history now.
Social media tie-ins
Postbox could post to and pull updates from Facebook and Twitter inside the mail client, which no other mail app did then. That kind of integration faded across the industry, and Twitter itself has since been renamed X.
Topics
Topics were a way to tag emails by their content. They were optional and suited people who sort mail by project more than by folder.
Composing
Writing an email was simple. Icons to the right of the compose window let you add files, links and images. Click the link icon and Postbox listed every link from every email on your machine, ready to drag into your message. Attachments, images and contacts worked the same way.
Memory use, price and platforms
Postbox used far more memory than Apple Mail. In the 2009 test it took about 210MB, compared with 32MB for Apple Mail, so it was a poor fit for older, slower Macs running OS X. It ran on Thunderbird’s Mozilla framework and was available for both Windows and Mac.
During the 2009 beta, Postbox was free to download. That changed with version 1.0 in September 2009, when it became a paid app with a free trial and a single-user license priced at $39.95. It stayed a paid product until it was discontinued in 2024.
What Postbox meant for email testing
The 2009 advice was to add Postbox to your list of test clients eventually, without rushing. Since it used Mozilla’s Gecko engine, it was expected to render HTML email much like Thunderbird, which was fairly forgiving at the time. Back then the hard case was Outlook 2007, which had switched to Microsoft Word for rendering HTML. That Word engine is still the reason classic Outlook for Windows breaks layouts today.
The advice now is simpler:
- Drop Postbox from your test list. It gets no security patches and no new releases. Backup vendor Druva dropped support for Postbox on macOS in April 2026, citing the vendor’s end of life.
- Test where your opens actually come from. Litmus’s email client market share data counts opens, not accounts. Apple Mail and Gmail lead it, Outlook comes next, and Thunderbird barely shows up. Apple’s share is partly inflated by Mail Privacy Protection, which preloads images and counts them as opens.
- Check mobile and dark mode. Most mail is now read on phones, and dark mode can invert colours, hide dark logos on clear backgrounds and wash out text. Every major client deserves a dark mode check before you send.
- Treat Thunderbird as a quick sanity check. It is the living descendant of the engine Postbox used. If your audience is technical, a test send to Thunderbird covers what Postbox once did.
- Fix authentication before rendering. A perfect design is worthless if it lands in spam. Google’s sender rules for anyone sending 5,000 or more messages a day to personal Gmail accounts took effect in February 2024, with one-click unsubscribe required from June 2024. Gmail began stepping up enforcement on non-compliant traffic in November 2025. Microsoft’s Outlook.com rules for domains sending more than 5,000 emails a day took effect on 5 May 2025.
A minimal DMARC record that meets the Gmail, Yahoo and Outlook.com baseline looks like this:
_dmarc.example.com. IN TXT "v=DMARC1; p=none; rua=mailto:[email protected]"
For bulk senders, p=none is the minimum. Once your reports show that all your legitimate mail passes SPF or DKIM with alignment, move to p=quarantine and then p=reject.
Where Postbox users went
The two main destinations were eM Client and Thunderbird, and the choice comes down to price versus polish. eM Client is the official path. It bought Postbox, offered migrating users discounted licenses until 31 December 2024, and has a free tier for personal use plus a paid Pro license. Existing Postbox licenses did not carry over. Thunderbird is free and open source and runs on Windows, macOS and Linux, with a separate Android app built from K-9 Mail. Since version 136 the desktop app has moved to monthly releases, and an Extended Support Release line remains for anyone who wants fewer changes. Mac users who want a lighter, keyboard-driven app often mention MailMate.
Postbox’s real legacy is the idea that desktop email should be searchable by content type: attachments, links, images and people. If that is what you miss, test search in each candidate before you think about anything else.
FAQ
Is Postbox still available to buy or download?
No. eM Client acquired Postbox in October 2024, sales and development ended right away, and user support ran only until 22 December 2024. The refund window for recent purchases closed on 22 November 2024.
Can you keep using an existing Postbox installation?
It may still open, but it no longer gets security updates, so it is a poor choice for any account you depend on. Backup service Druva dropped Postbox on macOS in April 2026 because the vendor had reached end of life. Export or move your mail to a maintained client while the app still runs.
What is the best alternative to Postbox?
eM Client is the official successor and the closest to Postbox in features. Thunderbird is the best free option and shares Postbox’s Mozilla roots. Mac users who want something lighter often choose MailMate.
Should email marketers test campaigns in Thunderbird?
Only as a secondary check. Litmus’s open-based data shows Apple Mail, Gmail and Outlook far ahead of Thunderbird. Give those three, plus mobile and dark mode views, the bulk of your testing time.
Why are open rates unreliable for Apple Mail users?
Since September 2021, Apple Mail Privacy Protection has preloaded remote images through Apple’s proxy, so many messages register an open whether or not anyone read them. Judge performance by clicks, conversions and replies, and treat raw open rate as a rough trend at best.
What do Gmail and Yahoo require from bulk senders?
Since February 2024, senders of 5,000 or more messages a day to personal Gmail accounts must pass SPF and DKIM, publish a DMARC policy of at least p=none aligned with the From domain, offer one-click unsubscribe and keep reported spam below 0.3%. Google recommends staying under 0.1%. Yahoo set matching rules at the same time, and Microsoft applied similar ones to Outlook.com from May 2025.
Sources
- Google Workspace Admin Help: Email sender guidelines FAQ
- Yahoo Sender Hub: Sender best practices
- Microsoft Tech Community: Outlook’s new requirements for high-volume senders
- Microsoft Postmaster: Policies, practices and guidelines
- eM Client: eM Client acquires Postbox Inc.
- Thunderbird release notes
- Litmus: Email client market share