Can you attach an incoming/outgoing email to OF Task?
 

Is there any way to attach an outgoing or incoming email (regarding that task) to a Task in OF?

You want to do this on command, right? It is possible to set things up so any time you receive an email matching some set of parameters, you get a new task created in OmniFocus, but I'm not going to address that here unless that is what you are requesting.

So, you're reading your email in Apple's Mail program, and an email comes in that needs some action in one of your projects. The easiest way to get it into OmniFocus is to use the Clip-o-tron. You select the message in the Mail app by clicking on the line in the list of messages, then invoke your clipping shortcut, and depending on your configuration, you either get a quick entry window showing the new action (where you can edit it, assign project/context/dates/etc.), or it is deposited directly in your OmniFocus Inbox. In either case, the action name will be the subject line from the message, the text of the message will be included, attachments linked, and there will be a link to the original message that you can use to jump back to Mail and bring up the message.

If you want to attach this stuff to an existing action, simply copy and paste the entire contents of the notes field from the newly created action. To do this operation with an outgoing message, send the message, open your Sent Mail mailbox, and clip the message to OmniFocus.

Or drag and drop the message from Mail's message list to the OF task. This creates a hyperlink to the message in the task's Notes field.

You have to be careful with the drag and drop from the message list, however; if you are editing the target row in OmniFocus, drag and drop will insert text in the action name, rather than inserting a link in the note! Clicking on the row handle (the dot to the left) before doing the drag and drop will prevent this from happening.