I used to backup my Outlook emails using Time Machine. The de-facto backup tool, in Mac, would easily cover all my files well. However, with time, I noticed that few mails in the backup came up with headers only and no mail body. On the first thought, I thought, it is probably the Time Machine who is guilty. Later I discovered that the Outlook was the real culprit. Outlook would often download my headers, but not the mail body. As a result, Time machine never received an entire message. Time Machine is a big no for backing up emails, in spite that it is not the biggest culprit.
Still, here is the link
How to backup mails using Time Machine?
Looking out for the best way on how to backup outlook emails, I found a tool from Inventpure named Mail Backup X. It sends me warnings against partly downloaded emails. Hitting the Resolve the warning often helps to download the full content.
That is helpful.
How to backup Outlook Emails on Mac?