Thanks, yes I tried latest development build. I had communications with Kostya back in march regarding this, he was very kind to explain that it happens because of IMAP servers are allowed to do anything with special characters (accents) when they occur in message headers that aren't properly encoded.
Gmail IMAP servers replaces those characters with the non printable character (small question mark on a black diamond), and Aquamail never gets to see the original characters to recover it (it's a one way replacement).
He also pointed out that other mail apps (Gmail, K9, Outlook) use IMAP differently, where Aquamail is centered around the needs of a mobile app (being able to download attachments separately from the main message) and strangely enough it's related.
The most compatible app to read the messages has been Outlook (desktop never ever has shown the non printable character neither on Titles nor on body), then Gmail (but it changes the background color of the messages, for example from yellow to white), then K9 that shows correct colors, and characters but in very rare occurrences may display the non printable character (noticed it in 1 of over 5,000 messages).
I know 0 about developing an app, and maybe it's something absurd to ask but I would really like the possibility to have the ability to parse IMAP like Gmail/k9 do, or the way Aquamail does, on a electable way (toggle a settings option). My guess is it would require 2 parsing/downloading engines to then feed data to the display part.
Thanks for all the help and attention.