Unable to delete mail data

You may try some of these steps, please check after each step...

1. If the Mail app is still installed, go to "Settings" > "Mail" > "Accounts" and tap on your email account. Scroll down and look for "Mail" or "Mail Days to Sync." Set it to a shorter time frame to clear the cache and reduce the amount of stored data.
2. Open the Mail app again (if it's still on your device) and look for a "Recently Deleted" folder under the email accounts. Sometimes, when you delete emails, they are moved to this folder and aren't permanently deleted until you empty it.
3. If this is your corporate iPhone, it is important to reach out to the system administrator of your corporate. They will be able to assist you with any issues or concerns related to your device.
   1. However, if it is your personal device, there are a few things to consider. Firstly, check if you have any profiles installed by your corporate, organization, enterprise, or school management. These profiles are typically used to manage certain settings or restrictions on your device.
   2. If you do have a profile installed, and you no longer wish to keep it, you can follow the steps provided by Apple to remove configuration profiles on your iPhone. By doing so, you will eliminate the profile and its associated settings from your device. Install or remove configuration profiles on iPhone - Apple ...
   3. In case you need further guidance or encounter any difficulties during the process, it is recommended to contact the system administrator of the organization that provided the profile to you. They will be able to assist you in removing the profile or addressing any concerns you may have.
4. Sometimes, a simple restart can resolve temporary software glitches. Turn off your iPhone, wait for a few seconds, and then turn it back on.
   1. Restart the device. That's it.
   2. Still not solved --> Force restart iPhone - Apple Support
5. If the issue persists, you can try resetting all settings on your iPhone. This will not erase your data but will reset all personalized settings to their default values. Go to Settings > General > Transfer & Reset > Reset > Reset All Settings and confirm the action. Note that you will need to reconfigure your settings afterward. All settings—including network settings, the keyboard dictionary, the Home screen layout, location settings, privacy settings, and Apple Pay cards—are removed or reset to their defaults. No data or media are deleted.
   1. Reset All Settings: Return iPhone settings to their defaults - Apple Support:- Return iPhone settings to their defaults
   2. You can return settings to their defaults without erasing your content.
   3. If you want to save your settings, back up your iPhone before returning them to their defaults. For example, if you’re trying to solve a problem but returning settings to their defaults doesn’t help, you might want to restore your previous settings from a backup.
6. Make sure you have the latest updates installed.
   1. Update Over-the-Air (OTA) —> Update your iPhone or iPad - Apple Support (IN)
   2. Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If none of the above steps resolve the issue, it is recommended to reach out to Apple Support or visit an authorized service center for further assistance. They can provide specific guidance based on your device model and iOS version.

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.

You forgot to mention which email account you are referring to. Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other

It also depends on whether you have IMAP or POP3 configuration. iOS mail by default uses IMAP.

When you set up an email address in an email client, you will need to decide if you want to set it up as POP3 or IMAP. Both of them are ways to connect to the mail server so you can read your emails through an email client.

• IMAP is short for Internet Message Access Protocol. With IMAP, the message does not remain on the local device, such as a computer, it remains on the server.
• POP3 is short for Post Office Protocol. With POP3 mail, it will connect and attempt to keep the mail located on the local device (computer or mobile).