Contact Address (Castellano)

Using formatted text (eg HTML.) Or the irritating flowed format to allow plain text automatically adjust to the window size, imitating HTML behaviour. Among other important reasons the main is these formats break quoted text.
Doing top-posting, specially with email clients not able to quote (eg Outlook) letting original messages accumulate at the bottom as garbage on each response.
Appending noisy legal notes (won't protect your message neither at the practical nor at the legal level.)
Relying on email as a tool to send files (instead of a server,) abusing of attachments.
Not reading other's message before answering (forcing the other to repeat themself through twelve messages when the first was enough.) Or using software purposely designed to obstruct or even break the dialog as the one pointed in the following item.
Using annoying ticket systems, as large companies use today as part of their bureaucracy to discourage their clients from asking questions or support.
Forcing the user to suffer web forms to send messages, as those for contact in web sites o those used by the mentioned ticket systems. All fake privacy measures.
Using web mail interfaces (gmail, hotmail,) limited by design, or smartphones (both should be used only when there's no other option available.)