MacBook Mail has forgotten my iCloud email address from its list of "send new messages from"

MacBook Mail has forgotten my iCloud email address from its list of "send new messages from". This is my default personal email. It isn't a new account. When I go to compose a new email, or even reply to one already in my iCloud inbox, it defaults to one of my other two accounts to send from. The drop down list in the email "From" heading doesn't even include my iCloud address, and I can't manually type it in, either. Going to Mail -> Preferences -> Composing -> Send Messages From, doesn't show my iCloud address as an option, and choosing "automatically select best account" doesn't help (it was already set there). I still get emails to my iCloud inbox normally.