I'm here to ensure you can send your invoices to your customers, @jennifera2.
I've checked and found that this is a reported case. Rest assured that our engineers are working to fix this issue about customers being unable to receive emails.
In the meantime, let's download the invoice as a PDF and send it manually from the Email provider direct.
I suggest contacting our Phone Support Team, so you'll be added to the list of affected users and receive an email notification once we resolve the issue.