How to Migrate T-Online Emails to Office 365 (2026 Guide)

Written By Rohit Singh
Updated on October 15th, 2026

ⓘ Fact-checked and reviewed in accordance with our Editorial Policy

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A T-Online address often outlasts several jobs, laptops and phones. The mailbox behind it ends up holding years of invoices, contracts, travel bookings and family photos. Then one day the storage warning appears, or your employer moves everyone to Microsoft 365, and you have to decide what to do with all of it.

This guide shows you how to migrate T-Online emails to Office 365 in three ways: through Outlook, through the Exchange admin center, and with a Cigati IMAP Backup Tool. It also covers the preparation that prevents most failed migrations.

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Table of Contents

Why People Migrate T-Online to Microsoft 365

Most users have one of these reasons:

Storage: Free T-Online plans offer very little space, and even larger paid tiers fill up over a decade of attachments. A Microsoft 365 mailbox typically starts at 50 GB, and some plans go to 100 GB.
Work tools: Microsoft 365 puts Outlook, Teams, OneDrive, Word and Excel in one account. A standalone mailbox can’t compete with that.
Security controls: Microsoft 365 adds multi-factor authentication, data loss prevention and threat protection. That matters if you handle client or financial data.
Closing the old account: If you are cancelling T-Online, you need your history somewhere safe first.
One inbox: Checking two mailboxes every day gets tiring quickly.

Plan limits change. Check T-Online’s and Microsoft’s current plan pages before you decide.

Before You Start: Prepare the Ground

Spend ten minutes on these checks. They save hours later. (Microsoft Learn)

Clean up the source mailbox: Delete spam and obvious duplicates so you don’t migrate clutter.
Use a stable connection: A dropped session in the middle of a large folder is the most common cause of missing mail.
Keep a backup copy: Export or back up the T-Online mailbox first, so you can recover if something goes wrong.
Generate an app password: T-Online uses a separate password for third-party mail programs. In the E-Mail Center, open the gear icon, then Alle Einstellungen anzeigen, then Passwörter. Use the Passwort für E-Mail-Programme option to create one. Your normal login password usually won’t work for IMAP connections.

Server Settings You’ll Need When You Move T-Online Emails to Office 365

Server	Port	Security
IMAP (Incoming)	secureimap.t-online.de	993	SSL/TLS
SMTP (outgoing)	securesmtp.t-online.de	465 (SSL/TLS) or 587 (STARTTLS)	SSL/TLS or STARTTLS

Note: Keep these handy, because every method below needs them.

Method 01: Migrate T-Online Emails to Office 365 Using Outlook

This is the free route. It suits a small mailbox with a few thousand messages at most.

Open Outlook & go to File >> Add Account. In the new Outlook, use Settings (gear) >> Accounts >> Add Account.
Enter your full T-Online email address and choose Advanced options.
Tick Let me set up my account manually, then click Connect.
Select IMAP as the account type.
Enter the app password you created, plus the IMAP and SMTP details from the table above.
Click Next and wait for Outlook to confirm the connection, then click Done.
Drag your T-Online folders into the Microsoft 365 mailbox in the folder pane.

Limitations When You Import T-Online Emails to Office 365

Large folders can take hours, and Outlook may freeze on heavy attachments.
Each account has to be handled separately.
There is no date filter and no duplicate check.
If the sync stops partway, Outlook doesn’t tell you which messages were skipped.
You get no report to confirm what was moved.

Method 2: Transfer T-Online Emails to Office 365 Via Exchange Admin Center IMAP Migration

If you manage a Microsoft 365 tenant, you can use the built-in IMAP migration. Microsoft’s own tooling pulls mail from an IMAP server into Exchange Online, so no local client is involved.

Create a CSV file with the columns EmailAddress, UserName and Password for each mailbox.
In the Exchange admin center, create an IMAP migration endpoint using T-Online’s IMAP server and port.
Start a migration batch and upload your CSV.
Monitor the batch status until it completes.

Before you move large mailbox, look at the Exchange Online limits for mailbox and message size.

This method moves email only. Contacts, calendars and tasks are not included. You also need admin rights and a clean CSV, and any mistake in the credentials means a failed batch. For a one-person migration it is often more effort than it is worth. (Microsoft Learn)

Method 3: Migrate T-Online Emails to Office 365 with Cigati IMAP Backup Tool

When the mailbox is big, you have several accounts, or you can’t afford gaps, an automated tool is the safer choice. The Cigati IMAP Backup Tool from Cigati Solutions is built for IMAP accounts such as T-Online. It can move mail directly into Office 365, and it can also save messages as PST, MBOX, EML, MSG, PDF and other file formats.

What It Brings to a T-Online Migration

Directly migrate IMAP to Office 365: You migrate from the T-Online IMAP server to Microsoft 365 without exporting anything by hand.
Batch migration with CSV: Move several mailboxes in one session instead of repeating the steps for each user.
Date filter: Move only a chosen period, such as the last three years.
Duplicate removal: Filter on email fields so repeated messages don’t clutter your new mailbox.
Folder hierarchy preserved: Your folders and subfolders arrive in the same structure.
Attachment control: Migrate with attachments, without them, or save them separately.
Email preview: Check folders and item counts before anything moves.
PST splitting: If you export IMAP to PST, you can split large files into smaller ones.
Free up server space: An option to clear migrated mail from the source server afterwards, which helps if your T-Online box is full.
Availability: From the official Microsoft Store, you can download the software.

Step-by-Step: Migrate T-Online Emails to Office 365

Download & install the Cigati IMAP Backup Tool >> launch it.
Add your T-Online account. Enter your email address, the app password, the IMAP server (secureimap.t-online.de), and port 993. For multiple accounts, load a CSV instead.
Preview the mailbox. Review the folders and message counts, then tick the ones you want.
Then, pick Office 365 as the destination >> sign in to your Microsoft 365 account.
Set your filters. Choose a date range, enable duplicate removal, and decide how to handle attachments.
Start the migration >> let the progress screen run until it finishes.
Check the destination mailbox in Outlook or Outlook on the web to confirm everything arrived.

Try the free demo version first. Running a small test migration lets you check the output before you commit to a full run. Look at the complete step-by-step guide to understand the workflow of Cigati IMAP Backup Tool.

Quick Comparison

Factor	Outlook (Manual)	Exchange admin IMAP	Cigati IMAP Backup Tool
Best for	A few thousand emails	Admin-managed bulk moves	Large or multi-account mailboxes
Date Filter option for selective data migration	No	No	Yes
Remove duplicate emails	No	No	Yes
Folder Structure	Depends on your dragging	Preserved	Preserved
Technical effort	Medium	High	Low
Admin rights needed	No	Yes	No

Tips for a Smooth Migration

Test with one folder first: Confirm the app password, server details and destination all work.
Migrate in off-hours: A quiet network connection reduces timeouts.
Don’t delete the source yet: Keep the T-Online mailbox until you’ve spot-checked the new one, including older folders and attachments.
Update your address elsewhere: Banks, subscriptions and newsletters still point to your T-Online address, so plan to switch them or set up forwarding.

Conclusion

You can migrate T-Online emails to Office 365 with Outlook if your mailbox is small and you have time to supervise it. Administrators with many users can use the Exchange admin center’s IMAP migration. For large mailboxes, multiple accounts or a deadline, the Cigati IMAP Backup Tool gives you filters, duplicate removal and preserved folders in one workflow. Whichever you pick, create the app password, back up first and test a small batch before the full run.

Frequently Asked Questions

Q.1 How can I move T-Online emails to Office 365 without losing any data?

Ans. Back up your T-Online mailbox first, then create the app password and use a method that preserves folders and attachments. With the Cigati IMAP Backup Tool, you connect the account, preview the folders, choose Office 365 as the destination and start the migration. Afterwards, compare folder counts in both mailboxes before you close the old account.

Q.2 Can I select a specific date range when moving T-Online emails to Office 365?

Ans. Yes. Outlook’s drag-and-drop method has no date filter, but the Cigati IMAP Backup Tool lets you set a start and end date so only mail from that period is migrated.

Q.3 Can I migrate my T-Online emails to Office 365 without any technical knowledge?

Ans. Yes. The Cigati tool uses a guided, step-by-step interface. You only need your email address, the T-Online app password & the IMAP server details listed above.

Q.4 Is the T-Online to Office 365 migration compatible with both Windows and Mac OS?

Ans. The Outlook method works on both Windows and Mac, because Outlook is available on both. The Cigati IMAP Backup Tool is also available for both Windows and Mac.

Q.5 Which T-Online IMAP and SMTP settings do I need for the migration?

Ans. Use these details whichever method you choose:
1. IMAP server: secureimap.t-online.de, port 993, SSL/TLS
2. SMTP server: securesmtp.t-online.de, port 465 (SSL/TLS) or 587 (STARTTLS)
3. Username: your full T-Online email address
4. Password: the separate “Passwort für E-Mail-Programme” created in the T-Online E-Mail Center, not your webmail login password
Ports 143 and 587 without encryption are not recommended, because credentials travel in a less protected form.

Q.6 Will my folder structure and message dates stay the same after migration?

Ans. It depends on the method. With manual drag-and-drop in Outlook, the structure depends on how carefully you move each folder, and one mistake can flatten a subfolder. The Cigati IMAP Backup Tool has an option to preserve the folder hierarchy. Whichever method you use, spot-check a few older messages afterwards, and compare folder counts between the source and destination to confirm nothing was skipped.

Q.7 Why did my German folder names create duplicates like “Gesendet” and “Sent Items”?

Ans. T-Online uses German system folder names, such as Gesendet (Sent), Papierkorb (Trash) and Entwürfe (Drafts). Microsoft 365 uses its own default folders. During migration, these can arrive as separate folders next to the Outlook defaults instead of merging with them. This is harmless, but it can look messy. Run a small test batch first to see how your folders map. If needed, move the messages into the standard folders afterwards and delete the extra ones.

Q.8 My Outlook freezes or the migration stalls on a large folder. What should I do?

Ans. Large folders with heavy attachments are the usual cause. Try the following:
1. Migrate one folder at a time rather than the entire mailbox.
2. Split the job by date range, for example one year per run.
3. Pause other applications that use the same network.
4. Avoid putting the PC to sleep during the transfer.
If you regularly hit this wall, the mailbox is probably too large for drag-and-drop, and a filtered, automated migration will be more reliable.

Q.9 Is there a size limit for emails or mailboxes in Microsoft 365?

Ans. Yes. Exchange Online mailboxes commonly start at 50 GB, & larger plans can reach 100 GB. The default maximum message size is also capped, and older mail with very large attachments can fail to import. Limits and defaults vary by plan and tenant configuration, so check your plan and your admin’s settings before migrating very large messages.

Q.10 What happens if I run the migration twice? Will I get duplicate emails?

Ans. Possibly. Running a migration again against the same destination folders can copy messages a second time. The Cigati IMAP Backup Tool includes duplicate removal based on email fields, which helps when you have to repeat the migration.

Q.11 Can I migrate several T-Online accounts at once?

Ans. Yes, with the right method. Outlook requires you to set up each account one by one. The Exchange admin center handles multiple mailboxes through a CSV batch file. The Cigati IMAP Backup Tool also supports batch migration through a CSV file, so you can move several accounts in one session. Whichever you use, double-check every email address and app password in the file before starting, since one wrong entry can fail that mailbox.

Q.12 Will new emails still arrive in my T-Online inbox after the migration?

Ans. Yes. Migration copies existing mail and does not redirect anything. Until you set up forwarding or tell your contacts about your new address, new mail keeps arriving at T-Online. Set up a forward rule or update your address with important senders before you close or stop checking the old account.

Q.13 How do I confirm the migration worked before I delete the T-Online mailbox?

Ans. Do these checks before deleting anything:
1. Compare total item counts per folder in the old and new mailbox.
2. Open a sample of messages from different years and check that attachments open.
3. Search for a few known messages by sender or subject.
4. Confirm the sent items and archived folders arrived.
5. Keep the T-Online account and a backup copy for a few weeks as a safety net.

About The Author:

Rohit Singh

Rohit Singh is a technology professional with 7+ years of experience specializing in email systems, Exchange Server, Office 365, MS Outlook, and data migration solutions. He creates clear, practical, and solution-oriented content to help users and IT professionals resolve complex technical challenges efficiently.