Problems with mail coding

I need some help with this.
I speak Spanish, so all our clients speak spanish too. I’ve rewrited the
templates to translate to Spanish, and characters like áóéíú or ñ are
replaced with different things in mails sent by the rt-mailgate. However
they are displayed correctly on the WebUI (even those submitted by mail).
I’ve checked RT_Siteconfig and have found nothing to configure this, neither
in rt-mailgate and the postfix configuration files.

Anyone knows how to solve this?
Thanks