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
