Written by Jennifer Walsh. Reviewed for technical accuracy by Shubham Dixit, Independent Expert in Email Forensics and Data File Conversion. Updated 12 October 2026.
Shubham is an independent external reviewer and not an employee of PCDOTS.
Quick answer. Microsoft 365 has a free IMAP migration built in. Create a licensed mailbox for every Kerio user, enable IMAP on Kerio Connect, then in the Exchange admin center add an IMAP endpoint and a migration batch with a CSV of users. It copies the mail. Calendars and contacts don’t travel over IMAP, so recreate them or use a tool that carries them.
On this page
- How do you migrate Kerio to Office 365?
- How do you migrate Kerio to Office 365 free with IMAP?
- What does the IMAP migration leave behind?
- How do you move Kerio to Office 365 with a PST import?
- How do you migrate Kerio to Exchange instead?
- When does a migration tool help?
- Which method should you use?
- Frequently asked questions
How do you migrate Kerio to Office 365?
You migrate Kerio to Office 365 with Microsoft’s own free IMAP migration, not a paid tool to start with, whatever a tool-first guide says. Kerio Connect speaks IMAP, and Microsoft 365 can pull mail straight from any IMAP server.
There are three routes, and which one fits depends on your size and what you need to keep. The free IMAP migration in the Exchange admin center copies the mail for an entire team. A PST import suits a few mailboxes or an archive. A migration tool carries the mail along with calendars and contacts, which IMAP can’t.
One honest point up front. The free IMAP migration moves mail folders only. Calendars, contacts and tasks stay behind, so plan to recreate them or use a tool that brings them across. Each route is below.
People move for familiar reasons. Microsoft 365 is hosted, it ties into Teams and the Office apps, and it drops the cost of keeping a mail server patched and backed up. Kerio Connect is capable, but a cloud mailbox is one less server to look after.
Kerio Connect reaches Microsoft 365 three ways, the free IMAP migration, a PST import or a tool that also carries calendars and contacts.
“Most guides jump straight to a paid product, but Microsoft 365 already ships a free IMAP migration, and Kerio Connect speaks IMAP, so for mail it just works. The catch I always flag is scope. IMAP carries the mail folders and nothing else, so calendars, contacts and tasks are left behind. On a small move I migrate the mail for free and rebuild the few shared calendars by hand. On a big move, where losing the calendars and contacts isn’t an option, a migration tool that carries all of it in one pass is worth it. Either way I migrate one mailbox first and check it before the rest.” Shubham Dixit, Data File reviewer (draft, pending approval)
How do you migrate Kerio to Office 365 free with IMAP?
You migrate Kerio to Office 365 for free with the IMAP migration built into the Exchange admin center. First set up three things, then start the batch.
Set up first
- The mailboxes. Add and verify your domain in Microsoft 365, then create a licensed mailbox for every Kerio user. The target mailbox has to exist before the migration.
- Kerio IMAP. In Kerio Connect, under Configuration, Services, enable IMAPS on port 993 and open the firewall so Microsoft 365 can reach it.
- The user list. Build a CSV with the header EmailAddress, UserName, Password, one row per mailbox. Reset passwords for the file instead of asking users for theirs.
With that ready, here are the five steps.
- In the Exchange admin center, open Migration, then Endpoints, and add an IMAP migration endpoint that points to the Kerio server on port 993.
- Go to Migration batches, select Add migration batch and choose IMAP as the type.
- Select the endpoint, then upload your CSV of users.
- Name the batch and start it. Microsoft 365 copies the mail in the background and keeps new mail in sync.
- Once the mail is across, change your MX record to Microsoft 365 and complete the batch.
Migrate one mailbox first as a test batch and check it before you add the rest. For a big directory, stage the users in a few batches so you can watch every one finish.
Enable IMAP on Kerio, create licensed mailboxes with a CSV, add the endpoint and batch, then change MX to cut over.
What does the IMAP migration leave behind?
The free route is mail-first, and knowing its edges up front saves a nasty surprise on cutover day.
Know before you start
- Mail only. IMAP migration moves mail folders. Calendars, contacts and tasks don’t travel, so export and rebuild them, or use a tool that carries them.
- Item and size caps. Microsoft moves up to 500,000 items per mailbox and skips messages over 35 MB, so very large or very old mail may need a second pass.
- No official Kerio export. Kerio Connect has no outbound migration of its own, so the move happens from the Microsoft side over IMAP, or with a tool.
For the calendars and contacts, a user can export them to a file and bring them in, or a migration tool carries them with the mail in one pass. More on that below.
How do you move Kerio to Office 365 with a PST import?
For a few mailboxes, or to keep an offline archive, the PST route fits. You export the mailbox to a PST, then import that PST into Microsoft 365.
Export every mailbox with Outlook, the full steps are in our guide to export a Kerio mailbox to PST. Then import the PST into Microsoft 365 with the Microsoft Purview Import service, which takes a network upload of the files and loads them into the target mailboxes. This route gives you a copy on disk as well as the cloud, handy for retention or a slow cutover.
How do you migrate Kerio to Exchange instead?
Moving to on-premises Exchange rather than the cloud follows the same shape. Exchange supports an IMAP migration too, so you create the mailboxes in Exchange, enable IMAP on Kerio and point the migration at the Kerio server.
The same mail-only limit applies, calendars and contacts need a separate move or a tool. If you’d rather not touch IMAP, export every mailbox to a PST and import it into the Exchange mailbox. For many mailboxes, a migration tool that writes to Exchange or Microsoft 365 saves the per-user setup.
When does a migration tool help?
The free IMAP migration is right for mail. A migration tool earns its place when you need the calendars and contacts too, when the directory is large or when a hands-off bulk move matters more than saving the licence fee.
PCDOTS Kerio Migration Tool connects to the Kerio store or over IMAP and writes mail, calendars and contacts into Microsoft 365 or Exchange mailboxes in bulk, with user mapping and repeat passes for new mail. Follow these four steps.
- Install and open the migrator, then add your Kerio mailboxes or connect over IMAP.
- Map every Kerio user to a Microsoft 365 mailbox.
- Select Office 365 as the target, with mail, calendars and contacts on.
- Start the migration, then sign in to Microsoft 365 to check the result.
The tool route, the Kerio store to Microsoft 365 in bulk, with calendars and contacts kept.
Related reading, our guides to export a Kerio mailbox to PST and migrate AOL to Exchange Online. Leaving another server? See migrate Zimbra emails to Gmail.
Free and mail-only, or a tool that carries the calendars and contacts too, that’s the real choice.
Which method should you use?
Use the free IMAP migration for mail, a PST import for a few mailboxes and a tool when calendars and contacts have to come too.
| Method | Best for | Good to know |
|---|---|---|
| IMAP migration | Mail for an entire team | Free, mail only, 500,000 items and 35 MB caps |
| PST import | A few mailboxes, an archive | Export to PST, then Microsoft Purview import |
| Kerio Migration Tool | Calendars and contacts, bulk | Windows, user mapping, writes to M365 or Exchange |
Not sure which row is you? Match what you need to the route below.
Match what you need to the route, mail for a team goes free over IMAP, calendars and contacts need a tool, a few mailboxes suit a PST import.
Key takeaways
- Microsoft 365’s free IMAP migration moves the mail, so you don’t need a paid tool for email.
- Create a licensed mailbox per user, enable IMAP on Kerio, add a CSV, then the endpoint and batch.
- IMAP leaves calendars, contacts and tasks behind, and caps at 500,000 items and 35 MB a message.
- For the calendars and contacts or a bulk move, a migration tool carries all of it in one pass.
Frequently asked questions
Can I migrate Kerio to Office 365 for free?
Yes, for the mail. Microsoft 365 has a built-in IMAP migration. Create the mailboxes, enable IMAP on Kerio, then add an IMAP endpoint and a migration batch in the Exchange admin center. Calendars and contacts need a separate move.
Do calendars and contacts migrate over IMAP?
No. IMAP migration carries mail folders only. Export the calendars and contacts and bring them in by hand, or use a migration tool that moves them with the mail.
How do I migrate Kerio to Exchange?
The same way. Create the mailboxes in Exchange, enable IMAP on Kerio and point an IMAP migration at the Kerio server, or export every mailbox to a PST and import it. Calendars and contacts still need a separate move.
What are the IMAP migration limits?
Microsoft moves up to 500,000 items per mailbox and skips messages larger than 35 MB. Very large or very old mail may need a second pass or a tool.
Do I need the Kerio Outlook Connector to migrate?
No. The Kerio Outlook Connector links Outlook to a Kerio account, it isn’t a migrator. You can download it from GFI, but the migration itself happens over IMAP or with a migration tool.
How do I keep an offline copy as well?
Export every mailbox to a PST first, then import that PST into Microsoft 365. You end with the mail in the cloud and a PST on disk for retention.
So how are you moving your Kerio server to Microsoft 365?
Migrating Kerio to Office 365 is free for the mail, because Microsoft 365’s own IMAP migration pulls it straight from Kerio Connect. Create the mailboxes, enable IMAP, add a CSV, then the endpoint and batch and change your MX to cut over. Calendars, contacts and a bulk move are where a migration tool carries what IMAP can’t. So how are you moving your Kerio server to Microsoft 365, the free IMAP migration, or a tool for the calendars and the bulk?
