Use SingleMainWindow=true

kmail-refresh-settings/org.kde.kmail-refresh-settings.desktop

@@ -94,6 +94,7 @@ Comment[uk]=Оновлення параметрів KMail
 Comment[x-test]=xxRefresh KMail settingsxx
 Comment[zh_CN]=刷新 KMail 设置
 Terminal=false
+SingleMainWindow=true
 StartupNotify=true
 X-DBUS-StartupType=Unique
 X-DBUS-ServiceName=org.kde.kmail-refresh-settings

src/data/org.kde.kmail2.desktop

@@ -200,7 +200,7 @@ Categories=Qt;KDE;Office;Network;Email;
 MimeType=x-scheme-handler/mailto;
 InitialPreference=10
 Actions=Composer;CheckMail;
-X-GNOME-SingleWindow=true
+SingleMainWindow=true
 
 [Desktop Action Composer]
 Name=New Message...