IMAPSync Mailbox Migration Between Email Providers

You’re switching email providers — Gmail to your domain, old host to iWebVault, Outlook to something cheaper. You don’t want to lose years of mail. The right tool is IMAPSync, a battle-tested utility that copies mail between IMAP servers preserving folders, dates, read/unread state, flags, and message integrity. This guide covers when to use it, how to run it, and the gotchas around large mailboxes.

When IMAPSync is the right tool

• Both old and new providers support IMAP (almost all do).
• You want to migrate the actual messages, not just forward future mail.
• You need to preserve folder structure (Sent, Drafts, custom folders).
• You want to keep historical dates intact, not re-arrive everything as today.
• You’re moving multiple mailboxes or doing this regularly.

NOT needed when:

• You only have a small mailbox and you can use desktop client drag-and-drop instead (Thunderbird makes this trivial).
• You’re abandoning the old email entirely and don’t need to keep messages.
• The provider offers its own migration service (Google Workspace’s Data Migration Service for incoming).

What you need before starting

• Source server credentials — IMAP hostname, port, username, password.
• Destination server credentials — same for new mailbox.
• Destination mailbox already created and ready to receive mail.
• If using Gmail as source: app password (Google blocks regular passwords for IMAP).
• A server / machine to run IMAPSync from (or our team via support ticket).

Running IMAPSync

IMAPSync is a Perl script. Most installations on Linux. Basic invocation:

imapsync 
  --host1 imap.old-provider.com --port1 993 --ssl1 
  --user1 you@old-domain.com --password1 'YOUR_OLD_PASSWORD' 
  --host2 mail.iwebvault.com --port2 993 --ssl2 
  --user2 you@your-domain.com --password2 'YOUR_NEW_PASSWORD'

That’s the minimum. IMAPSync connects to both, lists folders, and starts copying messages. Output streams to your terminal showing progress.

Common options worth adding

# Just test, don't actually copy (review what would happen)
--dry

# Resume after interruption (skip already-copied messages)
# Default behavior - IMAPSync detects already-copied messages

# Limit speed if your hosting is rate-limiting
--maxbytespersecond 500000   # 500KB/s

# Exclude specific folders
--exclude 'Spam|Trash|All Mail'

# Map folder names (Gmail has "[Gmail]/Sent Mail" vs cPanel's "Sent")
--regextrans2 's/[Gmail]/Sent Mail/Sent/'
--regextrans2 's/[Gmail]/Drafts/Drafts/'
--regextrans2 's/[Gmail]/Trash/Trash/'

For Gmail specifically, regex transforms map Google’s “[Gmail]/” prefix to standard folder names. Without this you end up with weird folder names on the new server.

Gmail-specific considerations

Get an App Password

1. Google Account → Security → 2-Step Verification (enable if not already).
2. App passwords → create new.
3. Pick “Other” device, name it “IMAPSync migration”.
4. Copy the 16-character password Google generates.
5. Use this as password1 in your IMAPSync command. Don’t use your regular Google password.

Skip Gmail’s special folders

Gmail’s “All Mail” is a virtual folder containing duplicates of everything else. Don’t migrate it — you’d duplicate every message:

--exclude '[Gmail]/All Mail'
--exclude '[Gmail]/Important'
--exclude '[Gmail]/Starred'

Star/Important labels in Gmail don’t have direct equivalents in standard IMAP anyway.

Timing for large mailboxes

Rough sync rates:

• 1,000 messages — 5-15 minutes.
• 10,000 messages — 30-90 minutes.
• 50,000 messages — 3-8 hours.
• 100,000+ messages — overnight job.

Highly variable based on average message size, attachment counts, and how aggressively each provider rate-limits IMAP. Gmail rate-limits aggressively to about 2,500 messages/hour outbound.

For huge mailboxes, run in batches by folder. Migrate Inbox first (most needed), then Sent, then others.

Cutover sequence — minimizing email loss during transition

The risky window is between “I’ve migrated old mail” and “DNS points to new server”. Mail arriving during this gap goes to the old server.

1. Set up new mailboxes on iWebVault. Test that you can send/receive directly via the new server’s IP.
2. Initial IMAPSync. Copy current mail from old to new. Can take hours.
3. Lower DNS TTL. On old DNS provider, change MX TTL to 300 seconds. Wait the original TTL (often 1 hour) for this to propagate.
4. Change MX records to point to iWebVault.
5. Run IMAPSync again — picks up any mail that arrived at old server during DNS propagation window.
6. Verify all mail arrived at new server. Compare message counts in both.
7. Final IMAPSync run after 24 hours to catch any stragglers.
8. Cancel old mailbox only after a week of confidence everything moved.

For business-critical migrations, accept some downtime (1-2 hours) by:

• Setting auto-responder on old account: “We’ve moved! Please resend to…”
• Doing migration during low-traffic window (Sunday morning).

Running IMAPSync without setting up a server

If you don’t have a Linux server to run IMAPSync from:

• Open a ticket with iWebVault. For new customer migrations we can run IMAPSync as part of onboarding.
• Online IMAPSync services. imapsync.lamiral.info offers paid migration runs.
• Use a VPS short-term. Spin up a $5 VPS, run IMAPSync, destroy it after migration.

Verifying migration succeeded

• Message count. IMAPSync output shows “Messages transferred” totals. Compare to source folder counts.
• Folder structure. Open the new mailbox; ensure all folders present.
• Date integrity. Open old messages; dates should match originals, not “today”.
• Attachments. Open a few messages with attachments to verify they came through.
• Search. Search for a known message; should find it.

Common issues

“Authentication failed for Gmail.” Need app password, not regular password. Also enable IMAP in Gmail Settings → POP/IMAP first.

“Gmail rate-limited after first hour.” Use --maxbytespersecond 500000 to throttle below Gmail’s limits. Run in batches across multiple sessions.

“Folder names look weird on new mailbox.” Need regextrans2 transforms for Gmail’s bracket-prefixed folders. See gmail section above.

“Some messages duplicate.” IMAPSync usually detects duplicates via Message-ID headers. If duplicates persist, source and destination have different Message-IDs (rare). Run with --useheader 'Message-Id' to enforce.

“My new mailbox quota fills up before sync completes.” Plan ahead — destination needs at least as much quota as source mailbox size.

What’s next

• Setting up mail clients with new mailboxes: Email on devices.
• Authentication for outbound deliverability after migration: SPF/DKIM/DMARC.
• Quota management on new mailboxes: Email quota guide.

IMAPSync is the gold-standard migration tool — preserves everything you care about, handles edge cases, and works across providers. The complexity is mostly in coordinating the cutover. For mailboxes you actively use, plan a careful sequence; for archived accounts, just run it once and move on.