We have identified an issue where some emails sent the evening of March 10 / the morning of March 11 appear to be stuck in a SENDING state, instead of being delivered. We have identified and resolved an issue where an internal network attached storage device became disconnected, causing some emails to get stuck in a SENDING... state, instead of actually being delivered. We are still investigating root cause further.

This issue has been resolved. All data syncs to Xero, QuickBooks Online, and Salesforce have caught up and fully synced.