Can't send invoices through

Hi Team, 

I've been trying to send invoices to clients but it seems like they don't go through. Every time I try sending it, a tab shows up and says 'Your invoice have been sent through Outlook', however, it does not show up in my sent folder nor can my clients receive the invoices. I've also tried to look at the invoice activity but it says if I used my email provider to send the invoice, the activity history is unavailable. 

Please let me know how can the invoices be sent through, thank you.