Bron Gondwana, CEO at Fastmail on the Fastmail blog in 2018:

The email in your mailbox is your copy of what was said, and nobody else can change it or make it go away. The fact that the content of an email can’t be edited is one of the best things about POP3 and IMAP email standards. I admit it annoyed me when I first ran into it – why can’t you just fix up a message in place – but the immutability is the real strength of email. You can safely forget the detail of something that you read in an email, knowing that when you go back to look at it, the information will be exactly the same.