Suppose you write an email. You have typed the address of someone in the "To" field incorrectly. For example, you may write "email@example.com". Before sending e-mail, you remember that this person spelled his name in a fun way. You want to delete the letter "H" in "John". The correct email address is "firstname.lastname@example.org"
In a large number of current e-mail clients, you can not edit an e-mail address already entered in the "To" field. Once you have pressed Enter or moved the keyboard focus to the body of the email, the "To" address changes. The UI changes what you have entered into something that can be deleted / deleted, but not modified. This is also true for carbon copy fields. If you type in someone's email address, you must completely remove the address and re-enter it. For many email clients, a left click on the email address may delete it, but does not enable text editing.
I'm talking about the interface, not what's hiding under the hood. The strange thing is that between 2000 and 2010, changing a destination email address was trivial. In most email clients, the "To" field was a text box. You can click anywhere in the "To" field and type in almost anything you like. The backspace key worked well. the delete key worked well; no matter what.
Features do not become popular in the user interfaces of many competing companies, unless these features are an improvement over the old method. There must be a reason to prohibit users from changing the email addresses previously entered in the "to" field. Technically, you can edit them by deleting them and copying them from scratch, but I hope my meaning is clear. What is the thought behind this? Is it a question of making "the common case" fast? Which user cases are faster / easier to use with the design style (new or with removal) instead of the old design style (text editing)?