Selecting this when composing an email allows you to edit the localpart entirely. Similarly, when replying to an email sent to your catch-all and you have an <s

Selecting this when composing an email allows you to edit the localpart entirely. Similarly, when replying to an email sent to your catch-all and you have an <star>@ identity for the hostname FastMail will automatically set your from address for you. I didn't know that, but it's fantastic news! I was doing things the awkward "gmail" way.