Migrate Fastmail to Outlook: IMAP Walkthrough for outlook.com

People mix up consumer Outlook and Office 365 constantly, so let's be clear about what this is. This walkthrough moves a Fastmail mailbox to a consumer outlook.com, hotmail.com, or live.com account — the kind you sign up for free at outlook.com, optionally with a Microsoft 365 Personal or Family subscription attached. It is not a guide for Microsoft 365 business tenants. The endpoint host is the same (outlook.office365.com) but the credentials, quotas, and admin model are different. Wrong destination, wrong walkthrough; if you're moving to a business tenant, stop and use the Office 365 guide.

Why this migration is usually simple#

Both ends speak straightforward IMAP. Fastmail's IMAP server is one of the most standards-compliant implementations in the business, and outlook.com accepts standard IMAP on the same frontend Microsoft uses for Exchange Online. Both require app passwords. Both preserve folder structure. There's no OAuth dance, no Bridge daemon, no encrypted-mailbox negotiation. What's left is mostly about quota, folder mapping, and verification.

The thing that usually gets people is the 15 GB free quota on outlook.com. Fastmail users tend to be longtime users with chunky archives; if your mailbox is over 15 GB and you don't have a Microsoft 365 Personal or Family subscription on the destination, the migration will fail with Quota exceeded partway through.

Before you start#

Four prerequisites:

• A Fastmail account on any paid tier with an app password generated for IMAP.
• An outlook.com (or hotmail.com or live.com) account ready as the destination, with 2-step verification enabled so you can create an app password.
• Enough storage on the destination. 15 GB free, 50 GB with M365 Personal/Family. Audit Fastmail's storage usage before starting.
• A workstation you can leave running for the duration of the job.

Step-by-step migration#

1. Audit the Fastmail mailbox size

   Sign in to Fastmail and open Settings → Account. The Storage section shows used and available. Note the used figure — that's roughly what the destination needs to absorb. Fastmail's stored figure is close to but not identical to what gets written to IMAP — expect IMAP usage on the destination to be 5–10% higher because of differences in how each system counts attachments and overhead. If you're over 13 GB on Fastmail and the destination is free outlook.com, upgrade the destination first.

2. Generate a Fastmail app password

   In Fastmail on the web, go to Settings → Privacy & Security → App passwords. The exact menu path occasionally shifts — search for "app password" in settings if you can't find it. Create a new app password named "outlook.com migration". Grant it IMAP & POP access only, not "All". Fastmail will display a 16-character password once. Copy it immediately; it cannot be retrieved later.

   For more on how app passwords fit into the auth landscape, the app password glossary entry covers the full picture.

3. Generate an outlook.com app password

   Sign in to account.microsoft.com with the destination outlook.com account. Open Security → Advanced security options. If you don't have 2-step verification enabled, turn it on now — without it, the App passwords section won't appear.

   Scroll to App passwords, click "Create a new app password", and name it "Fastmail import". Microsoft will display a 16-character password. Copy it. You can't retrieve it later either.

4. Configure the migration in Mailbox Taxi

   In Mailbox Taxi, create a new migration job. Source: Fastmail. Host imap.fastmail.com, port 993, SSL/TLS. Username: full Fastmail address (you@fastmail.com or you@yourcustomdomain.com). Password: Fastmail app password from step 2. Test connection.

   Destination: Outlook. Host outlook.office365.com, port 993, SSL/TLS. Username: full outlook.com address. Password: app password from step 3. Test that too.

   If either test fails with AUTHENTICATIONFAILED, the most likely cause is having pasted the account password instead of the app password. Re-copy from the respective settings page.

5. Map Fastmail folders to Outlook folders

   Mailbox Taxi scans both sides and proposes a folder map. The basics map cleanly: Inbox to Inbox, Sent to Sent Items, Drafts to Drafts, Archive to Archive, Spam to Junk, Trash to Deleted Items. Custom Fastmail folders become custom folders on outlook.com under the root with the same names.

   Nested folders translate cleanly. A Fastmail folder Projects/2024/Client A becomes a three-level nested folder on outlook.com. The IMAP folder delimiter on both sides is /, so no translation is needed.

   Things to consider adjusting:

   • Spam — usually exclude. outlook.com's spam filter will re-quarantine most of it anyway.
   • Trash — usually exclude. Pure waste of bytes.
   • Notes — if you used Fastmail Notes, it'll come across but outlook.com won't render it as notes.
   • Very deep nesting (5+ levels) — Outlook's web UI handles it but the mobile app gets cramped. Consider flattening.
6. Dry-run a single folder

   Pick one small folder — Drafts is good, or a project folder with 50–200 messages. Migrate only that one. When it completes, open outlook.com and verify subjects, dates, senders, attachments, and HTML render correctly. If anything's broken, fix it before the full run.

7. Run the full migration and verify

   Kick off the full job. Mailbox Taxi runs 4 connections in parallel by default, which is what outlook.com tolerates per account. Expect 60–90 minutes per gigabyte. Fastmail's IMAP doesn't throttle individual users much, so the limit is on the destination side.

   If you see Too many simultaneous connections, Mailbox Taxi will back off automatically. Persistent throttling means you should lower concurrency to 2.

   When the job finishes, Mailbox Taxi shows folder counts side by side. Confirm they match. Spot-check 20–30 random messages: open the original in Fastmail's web UI and the copy in outlook.com, confirm subject, date, From/To/Cc, attachments, and inline images all match.

Gotchas specific to Fastmail → outlook.com#

A few quirks that come up reliably.

Microsoft's spam classifier is aggressive on first delivery#

When messages arrive in an outlook.com mailbox via IMAP APPEND, Microsoft sometimes re-runs spam classification on them. Migrated messages that came from your Fastmail Inbox occasionally land in the destination's Junk folder. Users see this as "missing emails". Solution: after the migration, open Junk and bulk-mark anything that's clearly legitimate as "not junk".

outlook.com's threading is different#

Outlook's conversation view groups messages by subject and participants more aggressively than Fastmail's. Migrated messages may get visually re-grouped in unexpected ways. The underlying data is fine — sender, recipient, subject, body, headers — just the visual grouping changes.

Send-as for custom domains#

If you sent from a custom domain on Fastmail, outlook.com won't let you send from that custom domain out of the box. Consumer Outlook supports limited send-as via "Connected accounts" but the setup is fiddly. For a custom domain, you may need to set up POP-based fetching from Fastmail in outlook.com, or accept that send-as won't work cleanly on the consumer plan.

Mobile app reconfiguration#

After the migration, the Outlook mobile apps for iOS and Android use OAuth to sign in to outlook.com, not the IMAP app password you just generated. The app password is only for IMAP clients. Tell users this — they'll otherwise try to enter the IMAP app password into the Outlook mobile app and get confused when it asks for an account password instead.

Real errors you'll see and what they mean#

• AUTHENTICATIONFAILED on source — Fastmail account password used instead of app password.
• AUTHENTICATIONFAILED on destination — outlook.com account password used instead of app password, or 2-step verification isn't enabled.
• Too many simultaneous connections — outlook.com throttle. Mailbox Taxi backs off automatically.
• Quota exceeded — destination is full. You hit the 15 GB limit. Upgrade or exclude folders.
• Message too large for destination — outlook.com's per-message size limit is around 35 MB for IMAP APPEND. Old messages with huge attachments may fail. Skip these and handle manually.
• STARTTLS handshake failed — host/port misconfigured. Both ends use SSL/TLS on port 993.
• Folder UTF-7 conversion error — Fastmail folder has non-ASCII characters outlook.com won't accept. Rename on Fastmail.

Communicating the change#

For a personal or small-team Fastmail-to-outlook.com move, the comms are simpler than a business cutover but worth thinking through.

• Decide the cutover date and time.
• Set up forwarding from Fastmail to outlook.com so any inbound mail during the transition still reaches you.
• Update mobile apps, signatures, and any third-party services where you've registered the Fastmail address as your contact email.
• Decide how long to keep the Fastmail account alive as a fallback (30–60 days is typical).
• If you're a Fastmail subscriber moving to consumer outlook.com, set a calendar reminder to cancel the Fastmail subscription once you're confident.

Post-migration checklist#

• Folder counts match for every folder you care about.
• 20+ random messages confirmed visually identical on both sides.
• Fastmail filters reviewed and any business-critical ones recreated in outlook.com.
• Forwarding from Fastmail to outlook.com active during the transition.
• Mobile apps reconfigured.
• Send-as set up (if possible) for custom domains.
• Fastmail account kept alive for at least 30 days.