Fastmail is great, but this is not a reason. Gmail had this >10 years ago, and Thunderbird too.
So I just now ran a test, sent a mail that said "attached is a copy of your letter.", and Gmail sent it without comment.
But, when I enable the Javascript/Standard interface, it did remind me. I use the "Basic HTML" interface and never switched to the JS/Standard interface so I never saw the reminder. I tried the Standard interface recently and really don't care for it much, though I concede it has more features, like being able to select all messages in a mailbox. I had to use that to get all my Inbox mail moved the the Archive folder on Gmail when I migrated to Fastmail.