How to Fix QuickBooks Email Invoice Errors Quickly QuickBooks may fail to send invoices due to login credential mismatches or blocked email ports. Verifying SMTP details, updating passwords, and confirming secure email preferences often resolve the problem. Ensuring stable internet connectivity also improves successful transmission of invoices to clients. ------- https://www.apsense.com/article/877229-understanding-and-troubleshooting-quickbooks-email-invoices-not-working.html