Unfortunately, there is no such possibility. As a workaround, you can configure a synchronization between a custom multi-value attribute (e.g. CustomAttributeTextMultiValue1) and secondary SMTP addresses and allow users to modify the custom attribute. With this approach the users will be able to modify only the duplicates of the secondary SMTP addresses. The solution will work only in Adaxes, its maintenance will be quite difficult and the Email Proxy Addresses property updates performed via third-party tools will be displayed to the users with a delay. If the solution meets your needs, we will provide detailed instructions.