A while ago it was quite common that Outlook would always default to Port 25 rather than 465 for a secure outgoing (SMTP) server. If I remember correctly, to correct it the trick is to set or reset the encryption method to SSL/TLS, and then change port 25 to port 465.
I have not heard or seen that Port 25 default issue in Outlook for a while but perhaps it is still lingering on and it would explain the error you are seeing.😉
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