Glossary
Proxy Address
An alternative email address routed to the same Exchange mailbox, configured as a secondary SMTP alias.
A proxy address in Exchange Online is an alternative email address routed to the same mailbox — typically a secondary SMTP alias on top of the user's primary address. A user can have many proxy addresses for handling email under different identities: firstname.lastname@company.com (primary) plus flastname@company.com, firstname.lastname@oldcompany.com (from an acquisition), firstname@company.com (vanity), all delivered to the same mailbox. Managed in Exchange Admin Center or PowerShell (Set-Mailbox -EmailAddresses). The primary address gets used on outbound mail; proxies accept inbound. Distinct from forwarding (which sends mail elsewhere) — proxy addresses keep mail in the same mailbox.