We can use the Set-Mailbox cmdlet to modify the settings of existing mailboxes.
The EmailAddresses parameter specifies all the email addresses (proxy addresses) for the recipient, including the primary SMTP address. In on-premises Exchange organizations, the primary SMTP address and other proxy addresses are typically set by email address policies. However, you can use this parameter to configure other proxy addresses for the recipient.
To add or remove specify proxy addresses without affecting other existing values, use the following syntax:
@{Add="[<Type>]:<emailaddress1>","[<Type>]:<emailaddress2>"...;
Remove="[<Type>]:<emailaddress2>","[<Type>]:<emailaddress2>"...}.