When you email an invoice or estimate to an address that cannot be delivered to (a typo, a closed mailbox, an invalid domain), Manifold tells you. Within a few minutes of the bounce, the person who sent the email gets a bell notification naming the document and the address, with the reason the receiving mail system gave and a link to the invoice. The notification stays until it is acknowledged. Fix the address on the customer record, then open the invoice and click Email again.
Every invoice and estimate page has an Email History section below Payment History once the document has been emailed at least once. It lists each email sent from that document: when, to whom, who sent it, and the status: Sent (handed to the mail system, no word back yet), Delivered, Bounced, or Not delivered. A bounced row shows the reason underneath.
The same tracking covers emailed work orders, dealer statements, purchase orders, and vehicle condition reports; their bounces raise the same notification. What it cannot catch is a typo that happens to land on someone else's real mailbox, since that email is delivered normally. A quick look at the address before Send is still worth it.