Rmail reformats the header of each message before displaying it for the first time. Reformatting hides uninteresting header fields to reduce clutter. You can use the t command to show the entire header or to repeat the header reformatting operation.
Toggle display of complete header (rmail-toggle-header).
Reformatting the header involves deleting most header fields, on the grounds that they are not interesting. The variable rmail-ignored-headers holds a regular expression that specifies which header fields to hide in this way--if it matches the beginning of a header field, that whole field is hidden.
Rmail saves the complete original header before reformatting; to see it, use the t command (rmail-toggle-header). This discards the reformatted headers of the current message and displays it with the original header. Repeating t reformats the message again. Selecting the message again also reformats.
One consequence of this is that if you edit the reformatted header (using e; Section 29.15), subsequent use of t will discard your edits. On the other hand, if you use e after t, to edit the original (unreformatted) header, those changes are permanent.
When the t command has a prefix argument, a positive argument means to show the reformatted header, and a zero or negative argument means to show the full header.
When the terminal supports multiple fonts or colors, Rmail highlights certain header fields that are especially interesting--by default, the From and Subject fields. The variable rmail-highlighted-headers holds a regular expression that specifies the header fields to highlight; if it matches the beginning of a header field, that whole field is highlighted.
If you specify unusual colors for your text foreground and background, the colors used for highlighting may not go well with them. If so, specify different colors for the highlight face. That is worth doing because the highlight face is used for other kinds of highlighting as well. Section 13.1, for how to do this.
To turn off highlighting entirely in Rmail, set rmail-highlighted-headers to nil.
You can highlight and activate URLs in incoming messages by adding the function goto-address to the hook rmail-show-message-hook. Then you can browse these URLs by clicking on them with Mouse-2 or by moving to one and typing C-c RET. Section 31.28.2.