@FunkyB, you've ruled out all possibility of the issue being on the receiver’s end?
Do some emails go through whereas others seem to always be delayed?
What sort of gap are we looking at?
Sometimes minutes, sometimes hours. And it's not all of them. I will receive some emails immediately but some pop into the inbox hours after they were sent.
We have an shopping website which sends us an email when an order is made. It sends a copy to my personal account on gmail. When optus is having this issue my gmail account will receive the email immediately, but optus may take hours.