Migrate PST to Gmail: Convert and Upload to Workspace

A PST file showing up in a migration ticket usually means someone has an old Outlook archive on a laptop, a former employee's mailbox export, or a forensic copy from legal. Gmail and Google Workspace will not accept that PST directly. You need either a conversion step to MBOX or an IMAP-aware uploader that can read PST format and append messages folder by folder. This guide walks the practical route, including throttling, label mapping, and the failure modes that quietly drop messages.

Why PST does not import natively into Gmail#

Google Workspace has a data migration service for IMAP sources and a separate import path for MBOX through Google Takeout-compatible tools. PST is Microsoft's proprietary container and was never on the supported list. Workspace admins who try to upload a PST through the admin console get nowhere fast.

The two viable routes are:

• Convert PST to MBOX with a desktop converter or by re-exporting from Outlook on macOS, then use a tool that accepts MBOX and writes to Gmail over IMAP.
• Stream PST directly over IMAP with a desktop migration client that reads PST natively. This skips the intermediate file but still uses IMAP append under the hood.

Both paths end the same way: Gmail receives messages through IMAP. The difference is whether you keep an MBOX artifact for archival, and how much disk space you need during the move.

Prerequisites#

Before touching Gmail, get the PST into a known-good state. Half the support tickets I see are people fighting Gmail when the actual problem was a corrupt or password-locked PST.

Locate the PST file#

PST files live in different places depending on Outlook version and whether the user set a custom location:

• Outlook 2016 and later on Windows: C:\Users\<user>\Documents\Outlook Files\ and C:\Users\<user>\AppData\Local\Microsoft\Outlook\
• Outlook for Mac: there is no PST; the local cache is OLM. You will need to export to PST from a Windows machine first.
• Archive files (archive.pst): often pinned to a network share. Copy locally before doing anything.

Copy the file to a working folder on a local SSD. Never run conversion against a PST on a network share or a OneDrive-synced folder. File locking conflicts will corrupt the source.

Check file integrity#

Run the Outlook Inbox Repair Tool against the copy:

"C:\Program Files\Microsoft Office\root\Office16\scanpst.exe"

Point it at the PST. If it reports errors, let it repair and keep the .bak it creates. Repeat the scan until it returns clean. A PST that scanpst calls clean still might not be perfect, but a PST that scanpst flags will fail your migration.

Decide on conversion approach#

Three practical options:

1. PST to MBOX converter — Standalone tools that read PST and write MBOX. Output one MBOX file per folder.
2. Outlook plus IMAP uploader — Open the PST in Outlook, configure Gmail as an additional IMAP account, drag folders across. Slowest but no third-party converter needed.
3. Dedicated migration client — Reads PST directly and uploads to Gmail over IMAP. No intermediate file. This is what Mailbox Taxi handles for people who do not want to babysit a drag-and-drop.

For mailboxes under 5 GB and one-offs, option 2 is fine. For anything bigger or for batch jobs, options 1 or 3 are less painful.

Prepare the Gmail target#

You need either an app password or OAuth working against the target Gmail or Google Workspace account.

Enable IMAP#

In Gmail web settings, go to See all settings > Forwarding and POP/IMAP and enable IMAP. For Workspace tenants, the admin must also enable IMAP at the org level under Apps > Google Workspace > Gmail > End User Access.

Get an app password#

Personal Gmail accounts with 2-Step Verification on can generate an app password under Manage your Google Account > Security > App passwords. Workspace accounts often need OAuth instead because admins disable app passwords as a hardening measure.

If app passwords are blocked, you have two choices: ask the admin to issue a short-lived OAuth client for the migration, or use a tool that supports Workspace OAuth flow. For raw IMAP uploaders that only accept username and password, app passwords are mandatory.

Confirm quota headroom#

Workspace business accounts get pooled storage. A 10 GB PST landing in a tenant where most users are at 95 percent quota will fail with Message too large for destination or a hard quota error. Check Workspace storage in the admin console before you start.

Migrate the messages#

1. Stage the PST locally

   Copy the source PST to a working folder on the migration workstation. Confirm scanpst returns clean. Note the total size and folder list. If you are doing batch migrations, build a small spreadsheet with PST path, owner, target Gmail address, and approximate message count.

2. Choose conversion or direct upload

   For converter-based flow, run the PST-to-MBOX tool against the staged file. It writes one MBOX per folder, with sensible names. Verify the output folder mirrors the PST structure. For direct upload through a migration client, point the tool at the PST and the Gmail destination and let it enumerate folders.

3. Map folders to Gmail labels

   Gmail uses labels, not folders. Most migration tools handle this automatically: an Outlook folder named Clients/2024 becomes a Gmail label Clients/2024 because Gmail's IMAP layer interprets slash-separated names as nested labels. Confirm the mapping in your tool before starting. If you want a different naming convention, set it up here, not after upload.

4. Throttle for Gmail's IMAP limits

   Gmail caps a single account at about 750 MB of IMAP append per hour and 2,500 messages per day before it starts rate-limiting. For larger PSTs, plan multi-day runs or use a tool with built-in pacing. Concurrency should stay at 2 or 3 simultaneous folder uploads. Higher concurrency triggers Too many simultaneous connections and rolls you back.

5. Run the upload

   Start the migration during a low-traffic window for the user. IMAP append is roughly 80 to 120 messages per minute on a healthy residential connection. A 10 GB PST with around 50,000 messages will take 6 to 10 hours. Leave the migration running. Avoid putting the workstation to sleep; configure power settings to keep IMAP connections alive.

6. Verify after the run

   Open Gmail and check each top-level label against the original PST folder. Compare counts, not just spot-check. Most decent migration clients produce a log with per-folder counts and any skipped messages. Read the log. Re-run any folder that came up short.

What to expect during the move#

A few things will happen that surprise people doing this for the first time.

Flagged messages and read state#

Gmail preserves read and unread state from the PST. It does not preserve Outlook's flagged-for-followup state because there is no Gmail equivalent. Categorized colours are lost too. If those metadata matter, document the loss before you start so users do not file a ticket about missing flags.

Sent items#

The Outlook Sent Items folder maps to Gmail's Sent label automatically because the message headers identify the user as sender. Some tools double-write these and you end up with sent messages in both Sent and a label called Sent Items. Either disable the double-write or accept the duplicate label and clean up after.

Deleted Items and Junk#

Most migration tools skip Deleted Items and Junk Email by default. This is the right behaviour. If you genuinely need those messages, enable them explicitly in tool settings. Otherwise users get junk dumped into Gmail and complain.

Common errors and fixes#

AUTHENTICATIONFAILED#

The Gmail credential is wrong, IMAP is disabled, or the app password got revoked. Check IMAP is on, regenerate the app password, and confirm the username is the full email address.

OAuth2 token expired#

The migration ran longer than the token lifetime. Tools that handle OAuth properly refresh tokens silently. If your tool does not, you need to re-authenticate and resume from the last successful folder.

Too many simultaneous connections#

Gmail's per-account IMAP connection cap is around 15, but practical throughput collapses well before that. Drop concurrency to 2 or 3.

Folder UTF-7 conversion error#

A folder name contains a character that does not survive IMAP's modified UTF-7 encoding. Usually emoji or unusual punctuation. Rename the folder in the PST first, then re-export.

Message too large for destination#

Gmail caps individual messages at 25 MB for IMAP append. Larger messages will be skipped. The migration log lists them; you can extract attachments and re-deliver as separate messages if needed.

After the migration#

Reconciliation is where most migrations actually fail. People assume "it ran without errors" means "everything is there." Always:

• Compare message counts per folder.
• Spot-check the oldest message in each major folder.
• Verify large attachments survived.
• Search for known recent messages by sender and subject.
• Check Sent items for a recent outgoing message.

For deeper reference on the file formats you're handling, see the PST file glossary entry and the PST, MBOX, and EML migration guide.

If you are doing the same operation but the destination is Microsoft instead of Google, the PST to Office 365 and PST to Outlook walkthroughs cover the network upload and Outlook desktop import paths respectively. If you already have MBOX files instead of PST, jump to MBOX to Gmail.