Thank you for your reply - yes I thought the same as you, but the question and answer received from Powerdnn was as follows:
QUESTION: Hi, I am having problems with the site emails on www.ireland.ictennis.net. It seems that when a new user is added they are not receiving the welcome email from the system. Other messages such as role change and password reminder links work ok. I am using your mail relay service. Do you have any ideas why these particular emails are not getting sent ? Thank you
ANSWER: This is by design, due to the Spam Registration issue that plagued earlier versions of DNN. There are two options to circumvent the spam filter, the first is to use a third party mail relay for your site. We have a knowledge base article on how to perform this step here: https://support.managed.com/kb/a320/configuring-dotnetnuke-to-send-email.aspx
The other step would be customize the message. You can do this on Admin > Languages and edit the "EMAIL_USER_REGISTRATION_PUBLIC_BODY" entry after clicking on the edit button for System Resource. The filter is looking for the phrase "We are pleased to advise that you have been added as a Registered User to". Changing that will aide in getting past the filter.
I have now changed to an alternative SMTP provider and all the system emails are being sent without any problems.
The fundamental problem with this is that I was completely unaware that the emails were not being sent (i.e. no bounce backs etc) until my client noticed. Also only some system emails were not being sent.
I hope this helps others find a solution
Thanks again :)