I know nothing about Outlook, but is there maybe some kind of proxy-script that caches all images in all emails so that it is only fetched once for many readers?
When the people receiving your emails are all on the same internal networks and using a similar email client, you will find that the Unique Users/Unique Visitors KPI metric value will be wrong.
To solve this issue you want to append the parameter: &cid= which should be set to a uniquely generated string of exactly 16 hexadecimal characters (containing only characters 01234567890abcdefABCDEF). This will set the Visitor ID to a unique string for each email recipient ensuring Matomo tracks them correctly.