Solution for Invalid base64 string for MIME content when attempting to move mail to Exchange account
There are a number of posts regarding this issue going back to 2019.
The problem I was having is that we started using Microsoft Exchange (probably my first a mistake) and I wanted to transfer some old email residing in Mail.app (MBOX format) to a new Exchange Server email account. But for many emails I would get the error "Invalid base64 string for MIME content", and worse still it would corrupt those emails (always backup!)
The solution is as follows:
- Download em Client for macOS
- Install it and add your Exchange account
- Now go to em Client > Preferences > General > Show Local Folder - check the box then click on Save & Close
- Go to File > Import > Mail, then Next (don’t bother with the files mbox method, didn’t work for me)
- Choose “Import selected folders”, then Next
- Select relevant folders and give em Client Full Disk Access
- Choose to import data to Local Folders (root will do)
- Click on Finish and it will import
- Now you can right-click on the imported folders and simply move them to your Exchange email account.
