Different behaviour of vsto Outlook add-in in Outlook 2016 and Office 365

Question

I've created a vsto outlook add-in which works well in my Outlook 2016 application but some of it's features doesn't work in Office 365 outlook. For example, when mail send event is triggered, in outlook 2016, mail.Sender is null until that mail is sent but in Office 365, value is assigned before sending the mail. Similarly, mail.SentOnBehalfName is null in Outlook 2016 but it contains sender's Display Name in Office 365.

Is there information in form of webpage or something available highlighting what changes are made in new versions of outlook with respect to VSTO add-in(I searched but could not find one). Or is it on developer to test the plugin on various versions, debug to understand the differences and then find the solution for each outlook version?

Answer 1

Sender related properties are not guaranteed to be populated until the message is actually sent and moved to the Sent Items folder. This is not version specific.

If the sender is null, you can assume the default user account will be used ( Application.Session.CurrentUser ).