Should I Use a Separate Domain for Marketing Emails?

A client asks:

“We’ve been told that we should use a separate and unique domain for email that’s different from our main Web domain. Is this something you’d recommend?”

The most common business case for using a separate email domain is that doing so can help protect a company’s main Web domain (companyname.com) from spam complaints, and thus minimize the chance of that same domain being blacklisted. However, there are pros and cons to the approach, and it’s not a slam-dunk decision. (If it were, everyone would do it.)