Using older versions of Outlook for Mac for personal email and calendar
i tried the same, but switching to legacy outlook makes me lose all email account i have, meaning it is completely empty.
So i switched back to new outlook immediately.
But i think what also works is not sending emails with attachment immediately but wait for a minute.
Because sometimes you can see an attachment vanishing for a second inside the email and then reappearing.
My interpretation was, that an upload happens in the background and if it does not succeed before sending the email, the attachment will be lost on the way.
Did anyone else try some like that?