If you send a message (to yourself, say) with the word "From " at the beginning of a line, the composer (I think it is the composer) will add a ">" in front of it.

For example:

Send the following as the body of a message
-------------------------------------------------
Hello world<Enter>
From whatever
-------------------------------------------------

Results in:
-------------------------------------------------
Hello world
>From whatever
-------------------------------------------------

Where did the ">" character come from?

Note that the following are ok:
-------------------------------------------------
From
From: a
from a
-------------------------------------------------

If *must* be 
<beginning of line>From <word>