Hello,
I'm using Version 13 now and after one year I tried again to use SendMail. Same error. The incoming mail functions are working!
I walked again thru all the documentation and Community site. No inspiration. The failure message is very limited in information content.
Link to Telenet settings. The Wolfram Outgoing Preferences are:

In[80]:= Options[SendMail]
Out[80]= {MailAddressValidation :> Automatic,
MailSettings -> Automatic}
In[81]:= SendMail["*xxxx*@gmail.com", "Here is the body \
of the message."]

Some hints would be welcome.
Chris