Question : What is DMARC?

Learning about DMARC

DMARC aims to teach the receiving server the policy to adopt upon receipt of a message from a domain on which DMARC has been activated.
The receiving server will either accept or refuse the incoming message in the manner specified by DMARC's information and in the case of a refusal notify the specified domain.

Google has implemented a strict policy as to the origin of @gmail.com emails whereby an email sent by Mailpro signed by mailpro but whose "from" address is an @gmail will be rejected by the destination server because it does not comply with the conditions imposed by the DMARC's acceptance standard as initiated by Google.
(This also applies for: yahoo.com and yahoo.fr addresses)

Example:
A message sent from a Gmail address with Mailpro:
DKIM: DKIM from Mailpro
From: [email protected]
Return path: [email protected] (Mailpro)
This message will be rejected because the DKIM and return Path do not match the "From"

A message sent from Gmail:
DKIM: DKIM Gmail
From: [email protected]
Return path: [email protected]
This message will be accepted because the DKIM and return Path match the "From"

In light of the above we recommend not using @gmail.com addresses as a sending address for emailing. Choose an address specific to your company (e.g. [email protected]été.com) or while it is still possible, use a hotmail.com address.