Thanks Genreve,
This issue appears to have corrected now, the emails are all sending well again. I have this issues every now and then, previously I narrowed this down to hotmail address's only but now it seems to be on any random address's and at random times. It's common for me now to send a text to first time customers to ensure they monitor for the email/invoices but it's always the one customer I forget to text that ends up not receiving their invoice. I find it frustrating when this occurs, I don't know if the support team can do much as in the past I have been advised there is no issues they can find, and, the virtual assistant is useless at best.
I just struggle to monitor which emails are sending and which are being caught up and for what reason, which frustrates me as it all should be consistent and methodical.