Emails from one domain rejected as spam

Try this command: Set-ContentFilterConfig -BypassedSenderDomains [DomainToBeWhitelisted.com]

Example for multiples: Set-ContentFilterConfig -BypassedSenderDomains microsoft.com,woshub.com,gmail.com
Example for one e-mail address: Set-ContentFilterConfig -BypassedSenders safesender@gmail.com

I don't know if this is different than "safe senders" though.

That is the command I have used to add them to safe senders list.
It now appears that all emails coming from gmail are getting the same "treatment"

oh, are you trying to whitelist @gmail.com?  That seems like a bad idea since there are lots of gmail spam accounts out there.
Run this to see what your whitelist already has: get-ContentFilterConfig
look for this section for e-mails: "BypassedSenders"
look for this section for domains: "BypassedSenderDomains"

This may shed light on what is going on.  First, what is in there?  The specific e-mail you care about or the whole domain or neither?

Agree with Tim.  Before whitelisting anything, you need to find out what the problem is.  Domains can change hands, computers can be infected, a rogue user may try to blacken the company's reputation.

Find out why the emails are being rejected and then address that, one, specific problem.

I am not trying to whitelist gmail.
We have a partner hosting his domain email on google and I am trying to whitelist his domain xxxx.com
when I run the BypassedSenderdomain the only domain on that list is the xxx.com I have a problem with

in the reject e-mail does it confirm that Exchange is blocking it?  Like any reference to Microsoft Exchange?  I'm wondering if it is being blocked by a different spam filter (like maybe you are using a service and didn't realize it?  Like Mimecast, etc.)

It seems to be a little convoluted:
The Checkpoint firewall was sort of blocking the messages as spam, but regardless the messages were transferred to the exchange server where they were shown on the log as 'socket error' but the reply to the sender was spam error 554.

I agree, and have disabled the firewall spam filter.
The confusing part is that the "socket error" showed up on the exchange log.

So was there anything else you needed?

After disabling the Checkpoint firewall spam filter, Everything is working now.

Not quite sure why would Checkpoint block all emails from the particular domain but, all is good now. :-)
Thanks,