smtp.libero.it da MAC non funziona

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Try this: boot into Safe Mode according to How to use safe mode on your Mac and test to see if you can send emails. If so then you have a some 3rd party software installed that is preventing it. Then reboot normally, test again and post the results.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

You might want to delete and re-add your Libero account via settings>internet accounts to see if the issues persist. If the issues persist you might want to contact Libero support, it seems to be something on their side