Summary
Mailgun can't prevent an email client from displaying the "On Behalf Of" notation when the domains in the Sender and From fields don't match. To eliminate this notation, the same domain should be used in both fields.
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Additionally, message headers can be used to provide more information about the sender and the message.
This information can be used to verify the authenticity of the message and the sender.
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Summary
It is important to understand that there is no way for Mailgun to prevent an email client from displaying the "On Behalf Of" notation as long as the domains contained in the Sender and From fields don't match. However, ensuring the exact same domain is used in both fields will eliminate the "On Behalf Of" notation.
Why Do I See "On Behalf Of" in My Email? – Mailgun Help Center
mailgun.com
X-Mailgun-Incoming: Yes X-Originating-Ip: [2001:xxx:xxxx:xxx::beef:93] body-html: <div dir="ltr">Testing Mailgun's forwarded and stored message routes ...
Routes — Mailgun API documentation
mailgun.com
Authentication to the Mailgun API is done by providing an Authorization header using HTTP Basic Auth ; use api as the username and your API key as the ...
Introduction — Mailgun API documentation
mailgun.com
However, there are some good clues. Mailgun provides DKIM and SPF verification for incoming mail, which is shown in the MIME headers once spam filtering is ...
FAQ — Mailgun API documentation
mailgun.com