Configuring preferences and options in Outlook.com
ReynaldoB, this setting page is not just for setting external email sync.
There is also setting to change default "From" address (MS 365 Family - custom domain) but it is not accessible because of the "Unable to load..." error.
You can check answer on FAQ page:
Get a personalized email address in Microsoft 365
"How do I change the default "From" address used to send email with a personalized email address?"
- Go to your Outlook.com Email alias settings. <-- Link to Sync Email setting
- Set the default Fromaddressfrom the drop down of address available. <-- not possible, because of bug/error: "Unable to load these settings. Please try again later."