Edit: summarised the issue into my first post
Edit History
Hello - brief update. After going back and forth and investigating between phpmail and SMTP. The php option started working with alternate test email addresses for all 3 of the tests I mentioned above.
The strange issue is that they only work with non gmail.com email addresses. For the gmail test addresses, I have waited for them to appear and checked spam/ filters etc, but emails only seem not to work with gmail.
Any ideas? It's great to see the majority of emails are getting out, but gmail is quite popular. Is it a setting related to security/ spam perhaps?