All of these configurations are managed within the email client's server, either through a web browser or their dedicated apps—not within the iOS Mail app on an iPhone. Once you've set up these configurations on the email client's server, they will be synchronized and applied to iOS devices as well.
Which email account are you referring to? Google, iCloud, AOL, Exchange, Outlook, Yahoo or any Other (Refer to the pic )below


- Is this your corporate iPhone? Then please contact the system administrator of your corporate
- If it is your personal device, in that case, do you have any profile installed by your corporate/organisation/enterprise/School Management?
- Erase the profile added by you If you have installed a profile on your own --> Install or remove configuration profiles on iPhone - Apple ...
- Or please contact the system administrator of the organisation who provided the profile to you