Newton waits a few seconds before sending so you can undo, and scheduled emails wait until their time. If an email really didn't go out, Newton keeps it in Scheduled, marked as failed, so you can send it again.
Try these, in order
Wait for the undo delay. After you press Send, Newton holds the email for 5 seconds by default so you can undo it. On desktop you'll see Sending (5s)…. To change the delay, go to Settings › General › Undo send delay and choose Off, 5, 10 or 30 seconds. Closing Newton during the delay doesn't stop the email.
Look in Scheduled. Open Sent and click or tap … scheduled at the top. Scheduled emails, and emails that failed, wait there.
Send a failed email again. A failed email shows Send failed on desktop or Couldn't send on your phone. Use Send now to try again. If it says Reauth required, reconnect the account first.
Check your connection. Sending needs an internet connection, and Newton has no outbox. If Newton couldn't hand your email over, it opens the email again so you can resend it.
For IMAP accounts, check the outgoing server. Use port 465 with SSL/TLS, or port 587 with STARTTLS, whichever your provider lists. iCloud uses 587 with STARTTLS. Many providers also need an app password. To change these settings, add the account again with the same email address and the corrected outgoing server.
Good to know
Newton sends from its server, so a scheduled email goes out even when your devices are off.
Newton sends each email at most once. If it can't tell whether an email went out, it asks you to check your Sent folder instead of sending it twice.
When a scheduled email fails, Newton notifies you. On desktop it also appears in Pulse.