In order to trouble shoot issues with emails that do not behave how you would expect ie mail delivery problems, the support department will need to review the Full Headers for the problem email.
Please review the list that has been provided by our friends at MX Toolbox and choose your Mail Client so that you can provide us with the headers. Please copy and paste into an email and send it to our support team.
Click here https://mxtoolbox.com/Public/Content/EmailHeaders/ to review the list.
Remember to locate the headers for the email that is the problem. If you are the sender of an email and the receiving party has let you know that there was an issue with an email you sent them, please ask them to forward the headers of that email message back to you to send on to us. We will then investigate and advise further.
The header information in Mac Mail lives in the View Menu/messages/All Headers…