When sending an email to multiple recipients in Outlook Web App (Exchange webmail), the recipient addresses must be separated by semicolons.
Note that this is different from the Mirapoint (old) webmail, where addresses were separated by commas. If addresses are separated by commas, Outlook Web App will treat the entire series as a single email address, and will complain with “No Match Found”, giving only the option to remove the entire list and start over.
When an email is saved to draft or sent, Outlook Web App will attempt to validate the recipient addresses, displaying them in a light blue color and underlined. Sometimes, addresses in the Global Address List or in the user's contact list will be validated before the message is sent, depending on conditions including how long the email has remained open. In these cases, Outlook Web App may in fact be able to split the addresses out accordingly, even if they are separated by commas, but users should not rely on this behavior.