Hi @Dao Quang Dung ,
Good day.
Could you tell me the language you use to write the message?
And also are you using the desktop Outlook client or OWA? If so, what's the version?
Please check these articles and see if they could help:
https://support.microsoft.com/en-us/topic/email-message-body-is-garbled-when-the-russian-version-of-outlook-is-used-in-exchange-server-2016-52491f21-9dbb-766b-e2ac-7088c3563802
https://support.microsoft.com/en-us/topic/an-email-message-body-is-garbled-when-you-save-or-send-the-email-message-in-an-exchange-server-2010-environment-124005ea-b0f5-d28a-8292-f8dde0560cf5
The idea is to change the Preferred encoding to UTF-8, it's in File > Options > Advanced > Preferred encoding for Outgoing messages.
Best regards,
Lou
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