Outlook to Gmail Migration#

Migration Paths#

Assess the source environment first:

SourceBest MethodScopeOffice 365 / Exchange OnlineGoogle Data Migration Service (Admin Console)Org-wide, bulkOn-prem ExchangeGWMMO tool or IMAP migrationPer-user or bulkPST filesGWMMO tool or Outlook IMAP bridgePer-userOutlook.com (personal)IMAP migration in Admin ConsolePer-user

Pre-Migration Checklist#

Run through this with the client before touching anything:

1. Inventory — Number of users, total mailbox sizes, largest mailbox, shared mailboxes, distribution lists
2. Google Workspace plan — Confirm active subscription, verify admin access (Super Admin required)
3. Source admin access — Global Admin (O365) or Exchange Admin + app passwords
4. DNS access — Whoever manages MX records (registrar, Cloudflare, etc.)
5. Data scope — Email only? +Contacts? +Calendars? +Drive files?
6. Cutover window — When to switch MX records (weekend recommended)
7. Client communication — Draft email to users about the switch, new login instructions
8. Retention — Keep source mailboxes active 30 days post-migration as safety net

Method 1: Google Data Migration Service (Recommended for O365)#

Best for org-wide migrations from Exchange Online / Office 365.

Setup#

1. Google Admin Console → Data → Data import & export → Data Migration (New)
2. Select Microsoft Office 365 as source
3. Click Connect → sign in as Microsoft Global Admin → grant permissions
4. Select what to migrate: Email, Contacts, Calendars
5. Choose date range filter (optional — migrate only last N months to save time)

Execution#

1. Add users to migrate (CSV upload for bulk: source_email,destination_email)
2. Start migration — Google handles delta sync automatically
3. Monitor progress in Admin Console → shows per-user status
4. Migration runs in background — users can keep using Outlook during this phase

CSV Format#

csv

source_email,destination_email
john@company.onmicrosoft.com,john@company.com
jane@company.onmicrosoft.com,jane@company.com

See references/csv-template.md for full template with notes.

Method 2: GWMMO Tool (PST / Outlook Profile)#

Best for individual users or PST file imports.

1. Download GWMMO: https://tools.google.com/dlpage/gsmmo/
2. Install on machine with Outlook profile or PST access
3. Sign in with Google Workspace account
4. Select Outlook profile or browse to PST file
5. Choose data: Email, Contacts, Calendars, Junk
6. Start import — progress shown in tool

Limitations: Runs per-user, requires desktop access, Windows only.

Method 3: IMAP Bridge (Manual / Small Scale)#

For personal Outlook.com accounts or when other methods fail:

1. Enable IMAP in Gmail: Settings → Forwarding and POP/IMAP → Enable IMAP
2. In Outlook desktop, add Gmail as IMAP account
3. Drag-and-drop folders from Outlook mailbox to Gmail IMAP folders
4. Slow but works for any IMAP-capable source

DNS / MX Cutover#

After migration data is transferred:

1. Verify domain in Google Admin Console (TXT record)

2. Update MX records to Google: text

   Priority  Host
   1         ASPMX.L.GOOGLE.COM
   5         ALT1.ASPMX.L.GOOGLE.COM
   5         ALT2.ASPMX.L.GOOGLE.COM
   10        ALT3.ASPMX.L.GOOGLE.COM
   10        ALT4.ASPMX.L.GOOGLE.COM
   
   

3. Set SPF: v=spf1 include:_spf.google.com ~all

4. Set DKIM: Admin Console → Apps → Gmail → Authenticate email → Generate DKIM

5. Set DMARC: v=DMARC1; p=none; rua=mailto:admin@domain.com (start with p=none, tighten later)

6. TTL: Lower TTL to 300 before cutover, restore to 3600 after 48h

See references/dns-records.md for copy-paste DNS templates.

Post-Migration Validation#

Run these checks for every user:

• Email receiving on Gmail (send test from external)
• Historical emails present (spot-check oldest + newest)
• Folders/labels mapped correctly
• Contacts imported (check count)
• Calendar events present (check recurring events specifically)
• Shared mailboxes / groups recreated as Google Groups
• Signatures set up in Gmail
• Mobile devices re-configured (Gmail app or IMAP)
• Outlook desktop disconnected or reconfigured

See references/validation-checklist.md for printable per-user checklist.

Common Issues#

IssueFixMigration stuck at 0%Re-authorize source admin credentialsMissing foldersGoogle flattens deep nesting → becomes labelsCalendar recurring events brokenRe-create manually (known Google limitation with complex recurrences)Contacts duplicatedUse Google Contacts merge duplicates toolLarge attachments missingItems >25MB not migrated — export separatelyShared mailbox not migratingConvert to Google Group or shared driveMX propagation slowCheck TTL, use dig MX domain.com to verify

Deliverables Template#

For client-facing migration projects, prepare:

1. Migration Plan — Timeline, phases, user batches, rollback plan
2. User Communication — Email template announcing the switch
3. Admin Runbook — Step-by-step for the migration operator
4. Validation Report — Per-user sign-off checklist

See references/client-templates.md for ready-to-use templates.