How to stop read receipts in email?

If you have an email account sending read receipts, then it has nothing to do with iOS since iOS mail app does not support read receipt settings natively. That would have to be due to a setting on your email account, on the server side. If you control the account, login to your account and check the settings there. If you dont control the account, then talk to whomever does administer it.

Read receipts in all email systems are under the control of the account user, or the account admin if it is a centrally administered account (e.g. a corporate exchange account).

Note that for example, if using an MS exchange account, if you enabled read receipts in Outlook or Office365 settings on the web, then those would apply from all email clients you access the account from. You would then need to use a client (like Outlook) that supports changes to the account settings, or login to the account on the web and disable read receipts there.

Check with your company Office365 admin. While you may have turned them off, if the company has a policy of always having them on for internal mail, that global admin setting will override your personal account setting.

In iOS are you using Outlook or the iOS mail app, or something else?

Then it has to be something set up on your Office365 admin side. iOS mail does not provide any native support for altering those custom account settings. If you’ve turned it off on your Office365 account profile and it’s still sending them, then that is because someone has configured your enterprise MS Office subscription to behave that way.

Are you referring to emails or iMessages? If you mean email which email provider are you using?