how can i add diacritical marks to words in my email?
for example umlauts in German words, accent grave in French etc
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If you compose your messages using HTML then enable the menu in the Compose window and look for Insert|Symbol (or something like that; I don't have Thunderbird here in front of me right now to check the exact words used.)
If you use plaintext then get yourself the abcTajpu add-on.