How do I export Thunderbird inbox mail into my Microsoft Exchange Server Inbox?

After reading your issue, I have listed below a few of the best techniques for converting Thunderbird inbox mail into Microsoft Exchange Server inbox mail.

Export Emails from Thunderbird

• Install ImportExportTools NG in Thunderbird:
• Open Thunderbird.
• Navigate to Tools - Add-ons and Themes.
• Search for "ImportExportTools NG".
• Install the add-on and restart Thunderbird.
• Export Emails as EML Files:
• Right-click the folder e.g., "Inbox" in Thunderbird.
• Select "ImportExportTools NG"  Export all messages in the folder - EML format.
• Choose a location on your computer to save the exported emails.

Now, import emails into Exchange Server.

Use Microsoft Outlook Connected to Exchange Server

• Open Outlook and ensure it is connected to your Exchange account.
• Drag and drop the EML files into the desired folder e.g., "Inbox" in Outlook.

The above method may be hard and risky when you have a large amount of data. So, rather than using a risky way you can use a reliable tool like Gain Tools MBOX to PST Converter.

Now, Import the PST file into Outlook:

• Open Outlook.
• Go to File - Open & Export - Import/Export.
• Choose Import from another program or file - Outlook Data File .pst.
• Select the PST file and specify the folder to import into.

After completing the import, log in to your Exchange account using Outlook or another client to verify that all the emails are in the desired folder.

Due to various protocols & file formats, exporting Thunderbird inbox emails to Microsoft Exchange Server requires many steps. This task's specific steps are here:

Step 1: Thunderbird Email Export

• Add ImportExportTools NG:
• Launch Thunderbird.
• Select Add-ons and Themes from the menu.
• Install ImportExportTools NG from search.
• Restart Thunderbird.

Export Inbox:

• Right-click Thunderbird's Inbox.
• In ImportExportTools NG, select the Export folder.
• Choose EML or MBOX for export. For large email transfers, MBOX is best.
• Save the file: Decide where to store exported emails.

Second: Import emails to Exchange.

• For EML, use Outlook.
• Import EML Outlook:
• Create an Outlook Exchange Server mailbox.
• Just place EML files into this folder.
• Post to Exchange.

Second Choice: Third-Party Tool

Convert MBOX to PST:

Convert MBOX to PST MBOX to PST converter.

Outlook PST Import:

• Go to Outlook.
• File > Open/Export > Import/Export.
• Import from another software or file, choose Outlook Data File (.pst).
• Select the Exchange mailbox folder to import your PST file.

Exchange Sync:

After email import, Outlook syncs with Exchange instantly.

Step 3: Import Review

• Check your Outlook or Outlook Web Access Exchange Server inbox to ensure all emails were imported.

Notes: Please backup Thunderbird emails before proceeding.

Many third-party tools can convert MBOX to PST, especially BLR MBOX Converter. 

To export Thunderbird emails back to Microsoft Exchange Server, use Thunderbird's "ImportExportTools NG" add-on to export the emails as .eml files. Once exported, use a tool like "MFCMAPI" to import the .eml files into your Exchange inbox. This method avoids the size limit and restores emails for IMAP access.

To export Thunderbird mailbox to Microsoft Exchange Server inbox, first, use the "ImportExportTools NG" add-on in Thunderbird to export the mailbox as an MBOX file. Then, use a third-party tool like "MBOX Converter" to convert the MBOX file to PST format. Finally, import the PST file into Microsoft Exchange using Outlook’s Import feature or PowerShell. This process allows you to restore the mailbox content back to Exchange for IMAP access.