Have you tried testing the forms through Mailtrap? That will prove form submissions are leaving the site / hosting server. After that it's normally an issue with the mailbox – especially if you are using an external email system to send responses…
So although the update fixed this and we can now receive to this person's Outlook 365, there are still some Perch forms not finding their way through.
So some are working and some are not.
The client wants to know why and I'm trying to explain…