[Evolution] Why does evolution wrap lines after 50 characters?

[White_R <bruno anche no>]

Hi!
I'm using evolution 2.30.3 on debian squeeze.
Whet composing Plain Text messages, with Normal paragraph style (like
this), lines are wrapped after 50 characters. Why? does this behaviour
have anything to do with some rfc?

If I choose Preformatted paragraph style and then click on Format->Wrap
lines, lines are wrapped after 78 characters, which makes a lot more
sense.

Thank you for the attention,
Bruno