Best ask MYOB that one. I have no idea what methods they use to send mail, but checking their support forum, it appears they test with Microsoft Outlook. That is rather crappy testing, but something I have come to expect from accounting and line of business software. Thunderbird supports simple MAPI, and I think MYOB use that, but I also think that simple MAPI only works on 32bit windows. It is certainly not shipped with current versions of Windows and has to be downloaded from the Microsoft web site. https://www.microsoft.com/en-us/download/details.aspx?id=1004 I am aware that those that use Thunderbird need to place Thunderbird in offline mode before they send bulk invoices, as MYOB does not use send later and the speed it sends the invoices exceeds the speed with which they can be sent.