Q Apple’s Mail seems to send multiple copies of each message to my mail server. Even if I have only sent one email, its Activity box indicates that two were actually sent, although there is only a single message shown in the Sent folder. Is this because my work mail account is with Microsoft Exchange Server, or the result of a security hack?
A There do not appear to be any hacks or malware affecting Mail, so it is most unlikely to be anything sinister. Some excess activity may result from the protocols being used to connect to your server(s). The one setting which can sometimes cause confusion if you have inadvertently activated it is in Mail’s Preferences, under the Composing tool. The last item in the Composing section there allows you to automatically cc: yourself with each message sent. If that is enabled, when you send a message, Mail will automatically receive a copy sent back to it.
Plain POP/SMTP connections are simple and transfer single copies of incoming and outgoing messages, and that is it.
IMAP mail, as used for iCloud accounts, and those with Exchange Server, can be more complex as they keep messages locally and on the server. At times they spend a lot of time getting information from the server, and messing about with messages that have already been put into the Trash.
You would need to check the server logs to establish this, and should raise the matter with the Exchange Server system administrator.
Updated from the original, which was first published in MacUser volume 28 issue 15, 2012.