A recent update to OWA adds the option to allow users to choose which proxy addresses assigned to a mailbox they would like to send messages from. It’s a small change which completes the client support for the earlier server-side update to allow users to send using mailbox proxies, and it makes using proxy addresses more approachable and useful. OWA also includes a drop-down list in the compose message screen to allow users to select an address to send from, and makes sure that message headers are updated correctly so that messages go back to the right address.
Exchange Online supports the ability to send email using any SMTP proxy address assigned to a mailbox. Following the announcement of the feature, users had many questions including what clients can be used. Here are some common questions and answers about the feature, including some PowerShell to report the set of proxy addresses assigned to user mailboxes.