My company partner with a 3rd party company but we need to send some notification email to customer on behalf of this 3rd party company, let say 3rd party company is DEFI.COM and my company is XYZ.COM, I am thinking this way. I request DEFI.COM company to create a 2nd level domain name and mapping to my smtp server like this...
projectA.DEFI.COM ---> smtp1.xyz.com
the email address for notification sender email would be this notification@projectA.defi.com
Because this email address is for outgoing email purpose, not for receiving. Therefore; I think, there should not be any conflict, right?
The only thing is that this 3rd party company, just modify their DNS server.