Hey there! We'd need some more information in order to provide you with helpful advice about this particular issue. Can you let me know how the emails are being marked as spam? Is the sender getting a bounceback message, or are they being filtered into the spam box on your account?
If you don't have root access to the server you may need to work with your hosting provider or datacenter to have them check the mail logs to get more details.
Thanks for the additional details. If you only have cPanel access you may not be able to adjust the filtering that is making this happen, since that is set up at the root level of the server. The mail logs on the server would show the spam score that is being applied to the message, and from there you could adjust that to not mark the message as spam, but that's also something that would need to get handled by root.
I'm sorry I don't have more I can provide on my end, but reaching out to the host is the best thing to do in this situation.