Outlook uses the semicolon as the separator for multiple email recipients, but you can change and allow the comma to separate multiple message recipients when sending messages.
Step 1: Click the "File" tab from the Ribbon;
Step 2: Click "Options" from the left menu;
Step 3: Click "Mail" in the "Outlook Options" window;
Step 4: Move down and find "Send Messages" section;
Step 5: Check "Commas can be used to separate multiple message recipients";
Step 6: Click "OK" at the bottom to close the window.
