Customers not receiving invoices

HI I sent about 10 invoices out on the 26th Oct (Australia) and none of them were received by the customers. Even when I resent them a couple of days ago they still didnt receive them. some of the customers I have previously sent invoices with no trouble.
The program comes up saying that the invoice was sent.

Was there a glitch on the date or near that date that caused an issue? I have sent a couple of new invoices recently (in the last couple of days) and they seem to have been received.

now I am manually downloading and renaming each one so I can email them directly.