My wife and I both use MacBooks. Hers is newer (OSX 10.14.6), mine is elderly (OSX 10.11.6). Each of us uses email-accounts via Gmail and iCloud. If she sends me an email-message from either of her email-accounts to my Gmail-account, it is shown in her Sent folder, but not received by me. The same happens if she sends from her Gmail-account to my iCloud-account. However, if she sends from her iCloud-account to my iCloud-account, all goes well -- but ONLY so long as there is no attachment. Any help gratefully received, and of course my apology if I am in the wrong forum.