Hi Possum13,
Since you can send invoices to other clients without issues, it sounds like something on the recipient's end might be blocking the email, even if their IT team says otherwise.
A couple of things to try: Ask them to check their spam folder, see if their IT team can whitelist MYOB's email and double check their email address if it's correct or valid.
If it keeps happening, best to reach out to our live chat support through our virtual assistant MOCA or submit a case via MyAccount to further look into this issue.
Regards,
Sai