For Fastmail users who also use Mail.app, here’s a script that searches for the selected messages (in Mail.app) in the Fastmail web interface.

Why would you need such a thing? One reason is that Fastmail lets users specify an infinite number of aliases which they can give to an infinite number of websites (and subsequently block, if an infinite amount of spamming ensues). Mail.app doesn’t offer a good way to change your reply address, but replying from the Fastmail web interface does the trick nicely. (Thunderbird users can use the Virtual Identity plugin for this as well.)

Here’s the script:

--Searches for the messages selected in Mail.app using the Fastmail web interface
--By Dan Byler (http://bylr.net)

tell application "Mail"
    try
        set selected_messages to selection
        set remaining to count of selected_messages
        set the_url to "https://www.fastmail.fm/mail/?MLS=MS-*;MSS=!MB-*;SMB-CS="
        repeat with the_message in selected_messages
            set remaining to remaining - 1
            set message_id to the message id of the_message
            set the_url to the_url & "msgid%3A%22" & message_id & "%22"
            if remaining > 0 then set the_url to the_url & "%20OR%20"
        end repeat
        set the_url to the_url & ";SMB-SearchAll=on;MSignal=*P-1"
        open location the_url
    end try
end tell

Or download it here.