What is the safest way to merge Outlook PST files without losing emails or attachments?

Merging Outlook PST files can be completed manually or with a dedicated tool, according to your needs and the amount of data involved. The best way to stay safe is to always make a full backup of all PST files before you start.
The manual way
You can merge PST files manually via Outlook by following these steps

• Start Microsoft Outlook.
• Click on File, then Open & Export and then Open Outlook Data File.
• Find and open the PST file you want to combine.
• When the PST file shows up in the left panel, you can drag and drop folders like Inbox, Sent Items, and others into your main PST file.
• Do the same thing for other PST files.

Pros

• No need for third-party software.
• You have full control over the data you move.
• Safe if done with care.

Limitations

• Takes a long time for big PST files or many PST files.
• There is a chance of making a mistake (putting something in the wrong folder).
• Not great for PST files that are very big.
• There can be duplicate emails.

How to Use the GainTools Merge PST Tool

GainTools Merge PST Tool is a safer way to merge PST files if you have a lot of them or want to make the process easier.
You can do the following with the software

• Combine several PST files into one without changing the files that are already there.
• Keep the properties of the email and the folder structure.
  
• on't have the same item twice
  
• nstead of changing the source files, make a new PST file that combines them.

Why it is safer for large amounts of data

• Automated process reduces manual mistakes.
• Works well with big PST files.
• Keeps attachments, metadata and formatting.
• Offers a simple interface that is easy for non-technical users to use.

Last Suggestion
If you only have a few PST files and not a lot of data, the manual Outlook method works well. But for business use, large mailboxes or multiple PST files, the GainTools Merge PST Tool is usually safer, especially if you don't want to miss any emails or attachments.