I have a number of alias domains that are mapped to my primary domain. When I email using an alias domain from the Zoho Mail user interface, the emails are received without "On Behalf of" appearing.

However, if I use the Zoho SMTP server externally (from a Wordpress site) using the alias domain and authenticating with my primary credentials, the receipient gets the "On Behalf of" message appearing even though the messages appear in my Sent folder as normal emails. 

The message shows with primary@email.com on behalf of email@alias.com.

Is there a way to use the Zoho SMTP server externally without the "On Behalf of" message appearing. 

Thanks very much for any help you can provide.