When I submitted the add-in to Microsoft for review, I got this "Your add-in is not working when loaded in a Gmail account on Outlook for Mac. The minimum build number this supports is 16.9.1212." What does it mean and what can I do to make it support Mac? I do think it is anything that I can easily fix by changing the manifest file.
My add-in' client side uses Office.js's getCallbackTokenAsync to get the token and pass it to the back end. The back end then use the token to access the mail item and send it to an external application.
I found this blog Announcing add-in support for Gmail accounts in Mac Outlook saying that "You cannot use Exchange Web Services (EWS) to access items in the Gmail mailbox". It is this the reason? So I need to use Outlook rest API instead to make the add-in work in Outlook for Mac?
Yes to make the add-in work on Gmail account, you would have to make use of REST API calls and not EWS. while using getcallBacktokenAsync(), pass isrest='true' to get the REST token. The add-in can detect if the account is a gmail account by using the accountType.