We've just rolled out a new feature that should help users identify official FastMail emails and avoid fake phishing emails.
All future official FastMail emails should now have a green tick next to them in the mailbox listing and when viewing the email/conversation. They look like this:
Users should be careful of any future emails that claim to be from FastMail that don't have the green tick. These are almost certainly phishing emails that aim to steal your login details. Just report them as spam.
Note that the tick will only appear on future official FastMail emails, not existing ones. Also it only appears in the current web interface, not the classic web interface and not in external email clients (e.g. Outlook, Thunderbird, Mac Mail, etc)