Restart the device. That's it. If issue is not resolved... Force restart iPhone - Apple Support

Which email account are you referring to? Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other

If you can't send email from the Mail app on your iPhone or iPad, there are a few things you can try.
...
Check the Outbox for unsent email
- In Mail, go to your list of mailboxes.
- Tap Outbox. If you don't see an Outbox, then your email was sent.
- Tap an email in the Outbox. ...
- Tap Send.

If you can't send email on your iPhone or iPad - Apple Support
Check Mail Fetch and Notification settings
To adjust these settings: Go to Settings > Mail, then tap Accounts. Tap Fetch New Data. Choose a setting — such as Automatically or Manually — or choose a schedule for how often the Mail app fetches data.29-May-2021
If you can't receive emails on your iPhone, iPad or iPod touch