Open the order in WooCommerce → Orders. With Email Delivery active, the order's screen has a box, Emails about this order, listing the emails WooCommerce sent about it (the newest 50), whether each was for the customer or the shop, who it went to, when, and whether the mail server took it or what it said if it did not. Partly sent means the mail server took it for some of its addresses and refused the others, often a stale Bcc; the line under it names the ones it refused, and a line beside Send it again names who already has it, since pressing it sends them another copy. When the customer's own address was refused and only a Bcc of yours took it, the row says Refused. Open in the log shows the same message in the log.

To send one again, press Send it again on its row. It works for New order, Order details, Processing order, Completed order, Order on-hold, Cancelled order and Failed order, and sends WooCommerce's email for the order once more. An email to the customer goes to the address on the order as it is saved: if the customer's address was mistyped, correct it on the order and press Update first. New order, and the shop's own Cancelled order and Failed order, go to the recipients set in WooCommerce → Settings → Emails. One press sends one email; reloading the page sends nothing more, and a note on the order records the resend.

The box lists emails sent since the update that added it. Older ones are in the log, which you can search by the customer's address, pasted on its own or the way your email program copies it (Jane Customer <jane@example.com>). The search finds messages with that address on the To line, not in a Cc or a Bcc. Emails sent since the update also name their order in the log, by the number WooCommerce shows for it.

A customer says they never got their order email