専用の差出人アドレスを追加してみたけど...

403エラーですね。権限が無い？

"body": {
"status": 403,
"message": "You are not authorized to send mail on behalf of the specified sending account.\r\nclientRequestId: 60b82f61-1b36-4317-a4a8-060626bd151c\r\nserviceRequestId: 8b1c50f4-ba12-cc4b-1a93-7cfb97e97c5c",
"error": {
"message": "You are not authorized to send mail on behalf of the specified sending account.",
"code": "ErrorSendAsDenied",
"originalMessage": "The user account which was used to submit this request does not have the right to send mail on behalf of the specified sending account., Cannot submit message."
},
"source": "office365-je.azconn-je-001.p.azurewebsites.net"

協同所有者に入れてみたけど同じエラーです。

"body": {
"status": 403,
"message": "You are not authorized to send mail on behalf of the specified sending account.\r\nclientRequestId: 67e1c45c-c7d8-4dc5-811b-a6994e259954\r\nserviceRequestId: a39f95f5-213a-204c-bc44-e1c5c70a1ee5",
"error": {
"message": "You are not authorized to send mail on behalf of the specified sending account.",
"code": "ErrorSendAsDenied",
"originalMessage": "The user account which was used to submit this request does not have the right to send mail on behalf of the specified sending account., Cannot submit message."
},
"source": "office365-je.azconn-je-001.p.azurewebsites.net"
}

サイトのメンバーにも登録したけどエラーは変わらないですね。

以下の投稿を読むと「別のユーザーに代わって電子メールを送信する」ことができないと無理みたいだけど、どう設定するかは理解できない。

できないことが多すぎて、このツールは本当に使えるのか不思議です。