Migrate IONOS to Gmail: IMAP to Google Workspace Guide

IONOS works well for a few mailboxes and breaks down once you want shared drives, Meet calls, or chat threaded with your inbox. Moving to Google Workspace is the usual next step, but the IONOS side has no Google connector and Google's own Data Migration Service caps out at 100 mailboxes per batch with no resume on failure. That combination makes most IONOS-to-Gmail moves take twice as long as they should. This guide is the version of the process that actually finishes inside a weekend.

What you are actually moving#

IMAP mail and folders. That is it. Things that do not move over IMAP and need a separate plan:

• Contacts (export from each IONOS account as vCard, import into Google Contacts)
• Calendar (export as ICS from IONOS webmail, import into Google Calendar per user)
• Aliases (recreate as Google Workspace aliases on the matching user)
• Distribution lists (recreate as Google Groups)
• Filter rules (Gmail has filters but the syntax differs; rebuild manually or skip)
• Out-of-office replies (recreate inside Gmail's vacation responder)

Pull a clean inventory of all six categories from the IONOS Mail Basic or Mail Business panel before touching anything. Without that list you will discover an unmigrated alias the day after MX cutover and inbound mail will start bouncing.

The IONOS IMAP host varies by region#

IONOS sells under different country entities and the IMAP host changes accordingly:

• imap.ionos.com — global / United States accounts
• imap.ionos.de — Germany
• imap.ionos.co.uk — United Kingdom
• imap.ionos.es — Spain
• imap.ionos.fr — France
• imap.ionos.it — Italy

Port 993 with implicit SSL. Username is always the full email address. If you point at the wrong regional host you get AUTHENTICATIONFAILED with no clue that the host is the issue.

If a mailbox has two-factor authentication enabled, generate an application password from the IONOS account's Security settings. The regular login password cannot authenticate against IMAP once 2FA is active.

Provisioning Google Workspace#

In the Workspace admin console:

1. Verify ownership of your domain. Google gives you a TXT record to add to IONOS DNS. Add it under the domain in the IONOS Domains & SSL panel. Propagation usually takes under an hour.
2. Buy the right number of seats. Buy one more than you think you need, so you have room for the catch-all and any service mailboxes.
3. Create each user account. Match the primary address exactly to the IONOS source. Set a temporary password and tell the migration tool to use it.
4. Create groups for any IONOS mailing lists. Add the same members.
5. Pre-create Google Workspace aliases that mirror every IONOS alias. They will not start receiving mail until MX cutover, but they need to exist before then.

Decide your IMAP authentication into Gmail now. Two options:

• App passwords with IMAP enabled per user: turn on IMAP in Gmail settings for each migrated user, then have each user (or the admin acting as them) generate an app password. Tedious but works without OAuth.
• OAuth with a service account and domain-wide delegation: cleaner for bulk migrations; you authorise once at the domain level and the migration tool impersonates each user. This is what we recommend for anything over 10 mailboxes.

The migration itself#

1. Lower MX TTL a week ahead

   Edit the MX records in IONOS DNS and change the TTL down to 300 seconds. Save and wait. After 24 hours, resolvers worldwide will be honouring the shorter TTL and your eventual cutover window shrinks from one hour to five minutes.

2. Pre-stage the bulk copy

   Start the IONOS-to-Gmail IMAP sync. Users keep working on IONOS; new mail will sync in the delta pass later. Expect 1 to 2 GB per hour per mailbox, with parallel mailboxes running concurrently. A 50-user, 500 GB total migration usually fits in a Friday-evening start and finishes by Sunday morning.

3. Send the user comms

   Two days out, send a short note: when the change is happening, that their address stays the same, that historical mail will already be in the new mailbox, and which mail client setup steps they will need to follow.

4. Cutover the MX record

   On cutover day, open the IONOS Domains & SSL panel for the domain, edit DNS, and replace the existing IONOS MX entries with Google's five MX hosts:

   • ASPMX.L.GOOGLE.COM at priority 1
   • ALT1.ASPMX.L.GOOGLE.COM at priority 5
   • ALT2.ASPMX.L.GOOGLE.COM at priority 5
   • ALT3.ASPMX.L.GOOGLE.COM at priority 10
   • ALT4.ASPMX.L.GOOGLE.COM at priority 10

   Save. Check with dig MX yourdomain from a terminal a minute later; you should see the Google hosts.

5. Run the final delta sync

   Immediately after MX flips, run a delta pass from IONOS to Gmail. This catches any messages that arrived at IONOS between the start of pre-stage and the MX cutover. Usually 200 to 3,000 messages depending on traffic.

6. Update SPF, DKIM, and DMARC

   Replace the IONOS SPF include with Google's:

   v=spf1 include:_spf.google.com ~all

   Enable DKIM signing in the Workspace admin console under Apps, Google Workspace, Gmail, Authenticate email. Add the published key as a TXT record at google._domainkey.yourdomain in IONOS DNS.

   If you have an existing DMARC record, leave it. If not, start with v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.

7. Reconfigure clients

   Walk users through removing the old IONOS account in Outlook, Apple Mail, or their mobile client and adding the Google Workspace account. On most platforms the Workspace account auto-configures from just the email and password.

Gmail's quirks you should expect#

Gmail is technically IMAP, but it is also fundamentally label-based and conversation-threaded. Three behaviours to be ready for:

• Folders become labels. Your IONOS folder Clients/Acme ends up as a Gmail label with the same nested name. Messages can hold multiple labels in Gmail but only one folder in IONOS, so re-labelling after migration is fine.
• A message in multiple folders on IONOS will appear once in Gmail with multiple labels. Some migration tools handle this incorrectly and duplicate the message; check the tool's behaviour before running production.
• All Mail accumulates everything. Gmail's All Mail label holds every message regardless of folder. This is normal. Count against "All Mail" rather than per-folder for total-message verification.

Errors you will see and how to handle them#

• Too many simultaneous connections from the IONOS side. Drop concurrent workers per mailbox to five and the throttle clears. IONOS does not document the exact ceiling but it is around 10 IMAP connections per mailbox before they push back.
• AUTHENTICATIONFAILED against IONOS. Usually wrong regional host or 2FA enabled and you used the login password instead of an app password.
• Lookup failed on the Gmail side. The destination mailbox has not finished provisioning yet. New Workspace mailboxes can take 5 to 30 minutes before they accept IMAP traffic.
• Folder UTF-7 conversion error. Common on German and French IONOS tenants. The folder name includes accented characters and your tool needs IMAP modified UTF-7 support to translate them correctly.
• OAuth2 token expired on the Gmail side. Refresh tokens are valid for seven days for the migration scope; if a long-running migration paused for the weekend, the token may need refreshing before the delta pass.
• Message too large for destination. Gmail's per-message limit is 25 MB for sending and 50 MB for receiving over IMAP. Messages from IONOS over 50 MB cannot be moved over IMAP; export them to MBOX as a one-off.

How Mailbox Taxi handles this pair#

Mailbox Taxi runs on your laptop and treats IONOS and Gmail as standard IMAP endpoints. It maps IONOS folders to Gmail labels correctly, deduplicates messages that lived in multiple IONOS folders, throttles itself when IONOS pushes back, and keeps a per-message manifest so you can resume across days. If you would rather see the Microsoft side, the IONOS to Office 365 guide walks the same source towards Microsoft 365. For the source-agnostic IMAP-to-Gmail mechanics with no IONOS specifics, the IMAP to Gmail walkthrough is the shorter read.

Validation checklist#

Within the first hour after MX cutover:

• Send a test from an external Gmail to a migrated mailbox. It should land in the Workspace inbox.
• Send a reply outbound. Check headers at the recipient end; SPF should pass and the message should be DKIM-signed by Google.
• Run a quick dig MX yourdomain and dig TXT yourdomain to confirm DNS shows the Google MX hosts and the updated SPF.
• Spot-check three migrated mailboxes against the source: one heavy user, one light user, the catch-all. Label-count and All-Mail-count should match within 1%.

If any of those fail, the Google Workspace migration playbook has the deeper troubleshooting section, and the MX record glossary covers what an MX flip is mechanically doing.

Decommissioning IONOS#

Keep the IONOS mailboxes paid for 30 days as a fallback. Do not cancel the same week as cutover. After 30 days, you have:

• Verified all DNS records are settled at Google
• Confirmed user counts and folder structure are intact
• Caught any unmigrated alias or distribution list

Only then cancel the IONOS Mail product. The domain itself can stay at IONOS or move to Google Domains / Cloudflare separately. That is a different decision and does not affect the mail migration. For the broader strategy on phased versus single-cutover migrations, the complete email migration guide is the place to start.