If storage is full mail will not be stored on server / device , How to check the storage on your iPhone and iPad - Apple Support
Manage your iCloud storage - Apple Support
Else the mail application is not working , remove the mail account and once again add If you can’t send email on your iPhone or iPad - Apple Support
Remove your email account and set it up again
- On your computer, sign in to your email provider’s website. Make sure that all of your email is there or make sure that your email is saved somewhere other than on your iOS or iPadOS device.
- On your device, go to Settings > Mail, then tap Accounts.
- Tap the email account that you want to remove.
- Tap Delete Account.
- Add your account again.
If the steps in this article don't help, contact your email service provider for more information.
Note : you can look your mail settings Mail – Official Apple Support
Additional tip .
The mail application is corrupted , remove the accounts and once again add Add email accounts in Mail on Mac - Apple Support
If you are using secondary authentication account in which three answers for security questions , date of birth , rescue mail address and its password is added , and iCloud Keychain is set up in Mac , make sure you must have iCloud Keychain verification code in records as it will be required once you add gmail or whatever services used and iCloud account .
( a ) Open system preferences > iCloud > the box for keychain is checked , click on options , click on change security code .
( b ) Click on security code > enter new code in the fields and click on next , you have to enter the new code for the second time and click on next .
In the next window the country code is written , enter your trusted mobile number …….and click on done .
Again enter your Apple ID password to update your account settings and click on ok , hence the code is changed , so write down this code in a safe place .
Note : once the code is generated then remove mail accounts and once again add it , make sure must have correct credentials like Apple ID password , web mail address and its password in your records .
