Outlook web UTF-8 charset

Outlook on the Web (OWA) exhibits a nuanced behavior in its handling of character encoding. By default, messages are generated using ISO-8859-1, with a transition to UTF-8 only when Unicode characters outside the ISO-8859-1 range are present. At present, Microsoft does not provide administrators with a global configuration option to enforce UTF-8 encoding across all messages in Exchange Online (Office 365). The encoding process is managed automatically by the service and cannot be overridden through administrative settings.

https://www.codetwo.com/kb/how-to-globally-enforce-utf-8-encoding-for-owa/