An invoice email failed: what now

Only owners and admins can send; accountants, bookkeepers and the tax advisor see the log but no Send by email. The invoice must be finalized with its PDF stored, otherwise the button is missing (see the page on invoice files, Try again). One send runs at a time: while a send is Waiting to send or Sending the button is hidden, and a second request is refused with "This invoice is being sent right now. Wait for the outcome."
The list under Sending shows every attempt, newest first, with recipients, subject and status: Waiting to send, Sending, Sent, Failed or Undeliverable. The page checks by itself every few seconds. After temporary trouble the app queues the mail again on its own, up to six tries, before it shows Failed. The first successful send puts "Sent on ..." into Invoice history; from then on the invoice can no longer be voided.
Reasons on the customer side: "The recipient's mail server rejected the message." (usually Undeliverable) means the address is wrong or the mailbox does not exist: fix it on the contact or in Recipients and send again. "Some recipients were rejected." under Sent means the others got it; the rejected address is in brackets. If every address in Recipients is rejected, only CC and BCC copies went out and the invoice counts as not sent.
Reasons on the app's side: "The mail server could not be reached.", "The mail server offered no encrypted connection; nothing was sent.", "Signing in to the mail server failed." and "The mail server refused to send." come from the mail service the app sends through; you cannot fix them yourself. Wait a while and choose Send by email again; if the same reason returns, tell support. "When the send ran, the invoice was no longer open or its files were missing." means it was voided or cancelled meanwhile; check Invoice history. "The stored files could not be read." is worth another try later.
"The send was interrupted; it is unclear whether the email arrived. Please check before sending again." The app never resends this by itself. Ask the customer first, otherwise they may get the invoice twice. Next time tick Send me a copy (as BCC to my own address) so you have proof.
"This client has no contact email, so the customer's replies reach nobody. Add it in the settings." Open Settings, Company profile tab, Invoice contact, and fill E-mail; replies go to it. The mail is sent anyway. The same form may warn that company details for the mail footer (section 37a HGB) are missing; it also still sends, with an incomplete footer.
Recipients starts with the customer's email from the contact (empty if the contact has none). It takes one to ten valid addresses, CC and BCC up to ten each. Dunning letters use the same kind of row; if the contact has no email address the Dunning section of the invoice says "Not mailed: the contact had no e-mail address. The letter is stored." Add the address to the contact.