You will need to ask the clients to add your Sending IPs to their SPF records and/or ask them to safelist your sending IPs with their anti-spam solution ( At the gateway or client level)
That's really the only way.
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We have company who sends emails out on our behalf to our clients, however many of these emails are going to junk folders because of spoofing.
I doubt they are going to change the way they do it any time soon.
How can I make the emails they are sending on behalf of our domain compliant?
They might add records if I email and ask them politely or if I pay for a 3rd party service. What’s the easy way around the issue with out asking too much of them.
Keeping in mind they are possibly spoofing hundreds of clients email addresses.
You will need to ask the clients to add your Sending IPs to their SPF records and/or ask them to safelist your sending IPs with their anti-spam solution ( At the gateway or client level)
That's really the only way.