Which phone?
Which version of android?
Which carrier?
Which email provider?
Which email app?
If the message is in the outbox, the app thinks it has not been sent.
You can delete it, but if it really has not been sent, it will be lost.
View the outbox; tap the message as if to edit it.
Click the "send" button and see if it sends.
If you get an error, what is the exact wording of the message?