Problem: Users not receiving emails from some particular domains.
Probable reason: If the sender of the mail has not listed his IP in his email server SPF records, then his email would be rejected or marked as spam by the receiver. This is done to honor the rule set by the sender.
Resolution: Add the sender's IP in the SPF records of the server.
In the above image, you can see the SPF records of a particular domain (flockmail.com).
If the sender of this domain sends an email from any IP other than the above-mentioned IP's, their email will be rejected from the receiver's side as per the condition mentioned in the above image i.e. Fail(reject) all the emails sent from any other IP apart from the ones mentioned above.