Sorry to hear about your problem. Since you have given us no information on your account, we can't look and see what might be causing the issue. In response to your question:
I've 4 email accounts on Mac Mail. 2 are gmail, 2 are webhostinghub and the non-gmail accounts consistently have moments where they simply won't send emails using the WHH server. It's not consistent. Surely there are others with this problem?
There are a variety of reasons that emails fail to be delivered. We have an article that goes over the general reasons: What do I do if an Email I sent is Returned?
. As per the article, the actual cause may vary. Using Gmail as your basis for whether email should work or not, is not a good way to judge this because email servers may use different criteria or processes on whether an email is delivered, not delivered, or even delayed (all in the name of spam prevention).
When mail is NOT delivered, the mail system will generate an error and there is generally a bounceback message which gives the specific reason that email is NOT delivered. This is an industry standard process. Unfortunately, the means by which mail servers determine if mail can be delivered to a specific destination is not standard. This is because an email server may use a different spam prevention method/service from another email service.
I am a Mac user, and I have no problems sending emails from my test accounts (and I also use gmail for my own primary email accounts). In general, when there have been problems with delivery, it has stemmed from incorrect settings or blacklisted servers (due to spam prevention lists). In order for us to troubleshoot the issue, we would need an email address and a URL to start looking. The best thing to provide would be the bounceback message (if there was one). Sometimes this message may end up in your junk/spam folder.
Please understand that these posts are public. If you want to handle the matter privately, then you should contact our live support team via email/chat/phone. If you wish for us to continue investigating the issue, then please provide us further information.
Apologies again for the problems with sending the email.