First short We'll explain what you can do when mails are shown without content. Just as simple as this problem can also solve the error with the unsent e-mails. Because even here a forced restart should be the first choice, before trying out a more complex method.
Hold down the Home Tase and the Power Button at the same time until the Apple logo appears on your display. The restart works only up to the iPhone 6s. Here's how it works on other models: → iPhone 7, iPhone 8, iPhone X. After rebooting and with an existing internet connection (mobile or WLAN) open the mail app again and the e-mail should be sent automatically
Resend the "hanging" email
If the trick does not bear fruit with the reboot, then another try to send could help.
Open to it the mail app and return to the mailbox overview within the app (top left button), select the "Outbox" and tap the unsent mail, which will be displayed with a red exclamation point [Submit]
If this nudge is not enough, then you have another way to send the mail.
Delete the unsent email
Open the message in the Outbox and copy the contents to the clipboard. To do this, hold your finger briefly on the text field and select "All" from the context menu and then "Copy". Then tap "Cancel" on the top right, and then swipe your finger on the preview from right to left so that it will be deleted.
Depending on whether it was a new message or an answer, re-create the message and add a hint Once the message is ready, tap on "Send." Shipment should now proceed smoothly.
In some cases, however, only a poor Internet connection or an outdated password may exist for the mail server, the latter is always informed via pop-up.