When saving Email Defaults using Webmail an error shows: 

'Error with address ***@***.com. Check Windows event log for more details'.

We're currently investigating this issue. 

Workaround

Apply the email settings inside Report Designer manually.

If for example, it is an invoice you are trying to amend.

1. Click Invoices and credits.
2. Highlight an invoice and click Print.
3. Select a layout either from Favourites or Layouts.
4. Click Edit.
5. Make any necessary changes.
6. Click File, then Save All.