What are the main differences/annoyances you've noticed with Fastmail? Do you happen to know if they support "undo send"? i.e. where you can configure a 30-sec

What are the main differences/annoyances you've noticed with Fastmail? Do you happen to know if they support "undo send"? i.e. where you can configure a 30-second delay before the email is actually sent, so if you think of something right after you hit 'send' you can edit it. That happens to me a lot, so I like that feature.