Migrate Rackspace Email to Outlook: IMAP Transfer Guide

Plenty of small businesses landed on Rackspace's email products back when on-prem Exchange was unappealing and Office 365 wasn't quite ready. A decade later, Rackspace is no longer the obvious choice, and a meaningful share of those mailboxes are moving to Microsoft's mail products — sometimes outlook.com for individuals, sometimes Microsoft 365 for businesses keeping a custom domain. This guide covers the outlook.com path. The IMAP-side work is similar across destinations; the cutover sequence is what diverges.

Outlook.com versus Microsoft 365#

The word "Outlook" gets used for at least three different things. Disambiguating before the migration starts is worth a few minutes.

• Outlook.com — Microsoft's free consumer mail service at outlook.live.com. Hosts @outlook.com, @hotmail.com, @live.com addresses. Limited custom-domain support on paid Microsoft 365 Personal/Family plans.
• Microsoft 365 (formerly Office 365) — Microsoft's enterprise mail platform with Exchange Online. Hosts custom domains, supports shared mailboxes, distribution lists, and admin tooling.
• Outlook desktop — the mail client. Connects to any IMAP server or Exchange. Irrelevant to migration choice — you can use the desktop client against either of the above.

This guide targets outlook.com. For Microsoft 365 destinations with a custom domain, see our Rackspace to Office 365 guide and the broader Office 365 migration guide.

Identify your Rackspace source#

Rackspace sells two mail products under a similar brand:

Rackspace Email — homegrown, IMAP-only. Source:

• Server: secure.emailsrvr.com
• Port: 993, SSL

Rackspace Hosted Exchange — full Microsoft Exchange Server hosted by Rackspace. Source:

• Server: mex##.emailsrvr.com where ## is your cluster number (in admin console)
• Port: 993, SSL

Hosted Exchange exposes a few additional protocols (ActiveSync, EWS), but IMAP is universally available and the safest migration source. The migration tooling treats Hosted Exchange as just another IMAP server.

If you have both products on the same domain (some customers do), each migrates separately. Mailbox Taxi can run them sequentially in a single batch but the source endpoints are configured per-mailbox.

Pre-flight checklist#

Before launching:

• Confirmed Rackspace product type
• Mailbox password (or admin override password if you don't know the user's)
• Active Outlook.com destination account
• 2FA status on Outlook.com — generate app password if on
• Mailbox size estimate from the Rackspace admin console
• DNS access if you'll change MX records at cutover

Step 1: Create or confirm the Outlook.com destination#

For a fresh account, sign up at outlook.com. Choose an address you're happy to keep.

Confirm IMAP is on:

1. Sign in at outlook.live.com
2. Settings (gear), View all Outlook settings
3. Mail, Sync email
4. POP and IMAP section confirms IMAP is on

If 2FA is on (recommended), generate an app password:

1. Sign in at account.microsoft.com
2. Security, Advanced security options
3. App passwords, Create a new app password
4. Copy the 16-character string

Step 2: Configure the source#

For Rackspace Email:

• Server: secure.emailsrvr.com
• Port: 993, SSL on
• Username: full email address
• Password: Rackspace mailbox password

For Rackspace Hosted Exchange:

• Server: mex##.emailsrvr.com (your cluster from the admin console)
• Port: 993, SSL on
• Username: full email address
• Password: Exchange mailbox password

Test the source connection in a desktop mail client before configuring Mailbox Taxi.

Step 3: Configure the destination#

Outlook.com source for IMAP-bound migration:

• Server: outlook.office365.com
• Port: 993, SSL on
• Username: full Outlook.com email address
• Password: app password if 2FA on, otherwise account password

Step 4: Folder mapping#

Rackspace Email system folders:

• INBOX
• Sent
• Drafts
• Trash
• Spam

Outlook.com system folders:

• Inbox
• Sent Items
• Drafts
• Deleted Items
• Junk Email
• Archive

Map explicitly:

• Rackspace INBOX → Outlook.com Inbox
• Rackspace Sent → Outlook.com Sent Items
• Rackspace Drafts → Outlook.com Drafts
• Rackspace Trash → Outlook.com Deleted Items
• Rackspace Spam → Outlook.com Junk Email

User folders carry their names as-is. Outlook.com's underlying Exchange store has a few reserved characters (:, \, /) that Rackspace IMAP allows literally; the tool warns you if a folder name needs sanitising.

For Hosted Exchange sources, the system folder names match Outlook.com exactly because both run on Exchange. That makes mapping a non-issue for Hosted Exchange to outlook.com.

Step 5: Pilot one folder#

Pick Sent or a stable user folder of 200 to 500 messages. Run that first.

Verify:

• Final count matches between Rackspace and Outlook.com
• Sort order matches (confirms INTERNALDATE preserved)
• Read/unread flags carry over
• Three random messages open with attachments intact via outlook.live.com (not just the desktop client)
• No folder encoding errors

Step 6: Full transfer#

Concurrency for this pair:

• Rackspace source: 3 to 4 connections (handles parallel fetch well)
• Outlook.com destination: 4 connections (Microsoft's IMAP gateway tolerates more parallel writes than iCloud or Yahoo)

Throughput: 1.5 to 3GB per hour, faster than most other pairs because both ends are reasonably tuned for IMAP.

• 5GB mailbox: 1 to 3 hours
• 25GB mailbox: 5 to 10 hours
• 50GB mailbox: overnight

Microsoft's IMAP endpoint applies a per-user rate cap that kicks in at higher concurrency. If you see Too many simultaneous connections mid-run, drop destination concurrency to 2 and the run resumes cleanly.

Step 7: Verify#

When complete:

1. Folder counts side by side. Outlook.com should match Rackspace within a small margin per folder.
2. Spot-check by year — open messages from 2017, 2020, 2024 and confirm content.
3. Test sending and receiving on Outlook.com.
4. Confirm folder structure in outlook.live.com sidebar looks right.

If a count is off by more than 5 to 10 per folder, run a delta sync.

Step 8: Cut over forwarding or MX#

For outlook.com destinations with a personal address change:

Forward Rackspace to Outlook.com.

1. Sign into the Rackspace Cloud Office admin console
2. Open the mailbox settings, Forwarding
3. Forward all incoming mail to your outlook.com address
4. Save

Run forwarding for 60 to 90 days while you update senders.

For outlook.com with Microsoft 365 Personal/Family custom-domain support:

If you have Microsoft 365 Personal/Family with a custom domain feature, you can verify your domain in your Microsoft account and point MX records. This is a different flow to enterprise Microsoft 365 with full Exchange Online; for business custom domains, the Office 365 migration guide is the right reference.

Common failures#

AUTHENTICATIONFAILED on Rackspace: password with awkward special characters. Reset to something without backslashes, quotes, or dollar signs.

AUTHENTICATIONFAILED on Outlook.com: 2FA is on and the account password was used. Generate app password.

Too many simultaneous connections on either side: drop concurrency to 2.

Message too large for destination on Outlook.com: outlook.com caps individual messages at around 25MB attachment size. Larger messages are rare in personal mail but exist. Mailbox Taxi logs them; you'll need to handle them manually if they exist.

STARTTLS handshake failed on Rackspace: VPN with TLS inspection. Move off the VPN.

After the migration#

• Update senders with the new address
• Auto-reply on the Rackspace mailbox for 30 to 60 days
• Keep forwarding for 90 days to catch infrequent senders
• Export Rackspace contacts and calendar before final shutdown
• Revoke the Outlook.com app password from account.microsoft.com

If outlook.com isn't the right destination, our Rackspace to Gmail and Rackspace to Office 365 guides cover those alternatives. For source-side guidance applicable to any IMAP-hosted mail going into Microsoft 365 with Exchange Online, see IMAP to Office 365. The complete email migration guide covers DNS, SPF, DKIM, and DMARC sequencing across any provider pair.