Using the target blank feature on a link without notification to the reader that the link will open in a new tab or window is a violation of the US and many other government laws for web accessibility. It is not recommended for use at any time without a warning such as link (opens in a new window) like that.
In addition to violating the laws for web design standards, such usage breaks expectation of the experience of clicking a link. It can also make some users angry when they have multiple tabs open in their browser and they click a link and nothing happens, click it again, nothing happens, and they think the link is broken. Later they find multiple versions of the same web page open and they have to go through them all and close them, sending nasty vibes in your direction - so to speak. :D
Posting by email may be done with HTML as well as plain text. If you follow the specific format for writing the HTML link and use the warning, it will work just fine. As stated in Post by Email — Support you must be using an HTML compatible email program, one that allows editing of the HTML before sending the email would allow you to add the target blank plus the warning.
I hope this helps. Thanks.