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Warning: The “reflector” is still very much in testing stages, and may stop working at any time, or may undergo significant changes.
The reflector feature we’ve had for a while on the beta server is now available for testing on the standard production Options –> Define Rules screen. Reflecting is similar to forwarding email, but with a crucial difference.
Forward: A copy of the email is sent to the target account (e.g. your work). When you reply, the email comes from your work account address.
Reflect: A copy of the email is sent to the target account (e.g. your work), with the headers munged in a special way. When you Reply or Reply All at work, the email is sent back via the FastMail reflector, which unmunges the headers, and makes the email look like it came from your FastMail account (it also stores a copy in your Sent Items folder if the personality is set to do so).
The net result, the people you're having a conversation with only ever see your FastMail email address, never your work one. This is useful for people that like to be able to read and reply to some personal email at work (e.g. say to organise going out to dinner somewhere), but whose workplace won’t let them access the FastMail website, and also who don’t want to give out their work address to friends/acquaintances so they keep all their personal email at FastMail only.
The “Key” field should be just a few random letters/numbers you make up that’s used to avoid the reflector being able to act as an “open relay”.