The F=@ delivery agent flag (F=@) allows all addresses for a given delivery agent to be rewritten by the User Database (userdb). The F=i delivery agent flag either suppresses that rewrite for the sender envelope (if absent) or allows that rewrite for the sender envelope (if present). For example, consider mail from the user jane:

MAIL From:<jane>     SMTP envelope sender
From: jane           header sender

Now assume that a User Database entry such as the following exists:

jane:mailname       Jane.Doe

If the F=i delivery agent flag is absent but the F=@ delivery agent flag is present, the envelope-sender address will remain unchanged, but the header-sender address will be rewritten by the User Database:

MAIL From:<jane>     SMTP envelope sender
From: Jane.Doe       header sender

But if both the F=i and F=@ delivery agent flags are present, the envelope- and header-sender addresses will both be rewritten by the User Database:

MAIL From:<Jane.Doe> SMTP envelope sender
From: Jane.Doe       header sender