Recognize reply (Re:) and forward (Fw:) acronyms for several languages. Or at least allow to change these in the options. I have German language, but it is “Re:” instead of the German one “AW:”. But the English ones should still be recognized as I mail with German and English people.
For example, I write an email and get an answer, in the subject line the German Outlook added "AW: " at the beginning. Then I answer again, the subject line changes to “Re: AW: …”, then “AW: Re: AW: …”, “Re: AW: Re: AW: …”.
Another option to recognize such reply chains and to repair it to just one single reply acronym would be also nice.
The Thunderbird alternative “Postbox” is able to do all that.