I use regular expressions for this.
Believe you need to turn on full path capture for this to work properly.
It does not work with the "new outlook" app on Windows, but works fine with the regular "outlook" windows app.

The exact regular expression may vary depending on what language your outlook app uses.

I have a separate auto tag timeline for "Dialogue" and track mail, slack, teams etc dialogue in that timeline. I use regex to try and extract the person I was communicating with and the subject of the conversation. If possible. (it is not always possible)

For English language Outlook (windows)

Create an autotag and for group:"Outlook" and use the following regex to extract the subject.

#"(?i)\A(?:to|from)[^\w]+([^,]+),[^\w]+subject:(.*)\Z"

You can access the subject via the special keyword {{2}}
Can also access the sender or recipient via the special keyword {{1}}

If using another language than English, just change the "to" "from" and "subject" phrases to the terms that outlook uses for your language.

Sometimes outlook uses a different format for the sender or recipient, that is far more complex to parse correctly with regex. Subject is reliably simple to parse though.

I normally have several autotag rules, with the "stop after first matching rule" option turned on. That allows me to capture and group drafts (no subject or recipient) also.