Somebody handed you a .pst file, an old Outlook archive from a Windows PC, a departing colleague's mailbox, or your own mail from a job you left, and you need to read it on a Mac that does not have Outlook. Or you have an .ost offline cache pulled from a machine that no longer boots. This guide covers opening both directly in mailin, what the size limits are, and how to get messages back out in formats anything can read.
A PST viewer for Mac that reads the file in place
PST is the Outlook archive format and OST is the offline cache Outlook keeps for a connected account. Both are dense binary containers holding messages, folders and attachments. mailin opens them with its own native reader: no conversion step, no intermediate export, and no upload to anyone's server. You select the file, mailin parses it on your Mac, and the messages appear in the same interface as any other archive.
That native reader is memory-mapped, which is the detail that makes large files workable. Instead of reading a multi-gigabyte PST into RAM, mailin maps the file and reads the pieces it needs as it needs them. Version 2.0 raised the ceiling accordingly: PST and OST files up to 50 GB per file are supported. That is a hard limit, so a file above 50 GB is not something mailin can open today.
Opening a PST or OST file
- Copy the
.pstor.ostfile somewhere on your Mac. - Open mailin and drag the file onto the window, or choose it with the file picker.
- Watch the progress indicator. Large files take time, but memory use stays bounded.
Format detection is automatic, so you can drop a PST alongside an mbox or a folder of .eml files in the same import. If the archive you received came as a ZIP, unzip it first; mailin 2.0 does not extract ZIP files on its own.
Single Outlook messages that were saved individually use the .msg extension. mailin opens those too, one message per file, so a folder of saved .msg files from a shared drive imports the same way.
What you get once it is open
Once parsed, a PST behaves like any other mailin archive. Conversations are threaded using the Message-ID, In-Reply-To and References headers, so replies group together even if they were filed in different Outlook folders. The folder structure is available as a filter, alongside sender, domain, date range and attachment filters.
Full-text search runs against an on-device index that covers subject, headers and body. It supports Boolean operators, wildcards, regex and proximity queries, and results are ranked by BM25 relevance. If the PST is large, the year-sharded index means a search bounded to a date range only reads the shards for those years. The search syntax post covers the query language in detail.
The MIME parser handles multipart messages, base64 and quoted-printable encoding, and the common charsets. Some very old or non-standard encodings may not render perfectly, which is worth knowing if the PST is decades old.
Getting mail out of the PST
Reading is often only half the job. If you need to hand a message to someone without Outlook, or archive it in a format that will still open years from now, mailin exports each email as EML, PDF, TIFF or MSG. EML is the standard RFC 822 format that nearly every mail client understands; PDF is the right choice for something a person needs to read or print.
For bulk work, you can export a selection to MBOX, CSV, JSON, Markdown, plain text or portable HTML, and pull every attachment out in one pass with bulk attachment download. Contacts found in headers export as vCard and calendar items as ICS. The export formats guide explains when to use which.
One honest note: mailin does have a PST export, but it is experimental and may not produce a file that Outlook will open. If your goal is to move mail back into Outlook, treat that path as unreliable and prefer EML, MSG or MBOX.
Requirements and cost
mailin runs on macOS 14.6 or later on both Apple Silicon and Intel Macs, and the same purchase covers iPhone and iPad on iOS and iPadOS 17.6 or later. The Free tier opens files in every supported format, including PST and OST, up to 500 emails, with no account or payment details. Larger archives need the Personal tier; see the pricing section for current prices.
Because mailin only reads files you explicitly select and never contacts a mail server, the PST never leaves your Mac. There is no account, no telemetry and no third-party SDK in the app.
FAQ
Do I need Outlook installed to open a PST on a Mac?
No. mailin has its own native PST and OST reader and does not depend on Outlook being present.
What is the maximum PST size mailin can open?
50 GB per file for PST and OST. The reader is memory-mapped, so files up to that size do not need to fit in RAM.
Can I convert a PST to something Outlook-free?
Yes. Export individual messages as EML or PDF, or a whole selection as MBOX, which most mail clients can import.