Earlier today, QuickBooks Online users began reporting 502 Bad Gateway errors when they attempted to send E-Invoices. Around 3:00 PM ET, Intuit began investigating the problem to identify the cause and implement a fix.
Update: Intuit was still investigating the cause as of an update posted at approximately 3:30 PM ET.
Update: Just before 4:00 PM ET today, Intuit reported implementing a fix and monitoring the results. They also reported a substantial backlog of emails being sent through the system and that it would take some time for the backlog to be managed and things to return to standard processing.
Update: As of 5:10 PM ET, Intuit has reported that the issue preventing QuickBooks Online users from E-Invoicing and sending other QuickBooks forms has been resolved. Users are now able to access invoices and purchase orders and use the pay functions.