Traditionally a "Re:" prefix is added to a subject line when replying to an email. Some email applications use localized terms for this (like "SV:" in Danish or "AW: in German). Evolution can recognize these terms to avoid subject lines getting longer as the conversation continues.
Open the Terminal application.
Run the command gsettings set org.gnome.evolution.mail composer-localized-re 'AW,SV' (in case that you want "AW:" and "SV:" to be recognized).
You can also use the dconf-editor application to do this.
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