I spent 90mins on the phone and computer today with Microsoft Office 365. They were very helpful and changed a lot of settings within Outlook.
When we finished I thought all was well but I can't forward invoices from Quickbooks Desktop to email them and although most attachments now open some did not.
I am going to try Quickbooks support tomorrow and if that does not work I'm going to uninstall Kaspersky as I've read on the web that this can cause the problem sometimes.
I have also done a complete reinstall of Microsoft 365 Office.
