question on special chars

Hi all,

I am having trouble sending special chars in messages, as Portuguese
has such. So, a typical word would be "acentuação".

Some friends have complained that they receive this word as
"acentua=E7=E3o", that is, with coded chars.

I asked them for the original message and it came out that messages
with unrecognized chars have a line as

    Content-Type: text/plain; charset="us-ascii"; format=flowed

And messages that are properly recognized have

    Content-Type: text/plain; charset="iso-8859-1"; format=flowed
    Content-Transfer-Encoding: 8bit

(but to get that I had to insert a

    Content-Type: text/plain; charset="iso-8859-1"

line myself in the mail leaving my machine, and it was converted
into the two lines above).

The question is: which program is responsible for putting those
lines in the headers?  How could I enforce that the right lines are
used? Editing replcomps? (BTW, is that the right way of handling
the problem?)

thanks for any help,

joao b. oliveira