Quote Originally Posted by AmithS View Post
host smtp.ucebox.co.za [xx.xx.xxx.x]
That is the email server which has "blown the whistle" - probably yours given that it's an email to gmail that has not been sent.

You probably should talk to your email service provider about the spam detection settings for your account.
You can also play with the wording in the covering email, although the issue looks likely to be multiple emails being sent to the same email address in very close succession.