Imagine for a moment that you're a hiring manager and you've been handed a resume. You have approximately 2 minutes to decide whether to call this individual and conduct a phone screen. The resume can be a bit misleading. In some cases, it's a semi-reliable document that presents the candidate in his or her best light—great! When the resume has come from a recruiter, though, it's often NOT the resume the candidate provided. Many recruiters will format and "clean up" a resume, possibly introducing errors along the way. The same is true for a cover letter, if there is one. There is one small thing you can look at, though: the candidate's e-mail address. That's almost never changed—even when a recruiter has handled the resume—and it can tell you something about how the candidate chooses to present himself or herself. This is a generalization, of course, and you should take it with a grain of salt. However, it is part of the overall picture that the candidate paints in your hiring-manager mind. Here are some examples:

• A custom domain (e.g., foo@mycompany.com): Take a look at the website; this is what the candidate is most identifying with. This is also an indicator that the candidate in question has or had other interests. It's not a requirement—don't go out and register a domain just for this—but it doesn't hurt.
• Gmail (e.g., foo@gmail.com): This is a perfectly fine e-mail address. It's a current, widely used service. It's the equivalent of choosing to write standalone CSS instead of saas or less; it's not the hot new toy, but it's a perfectly acceptable solution.
• Hotmail or Yahoo (e.g., foo@hotmail.com): Think back to when these were popular, circa 2001-2007-ish. That's about when this person last chose to make a change in e-mail. Often, this coincides with the most recent technologies the person has worked with. It's fine if you're using some older technologies, but it should be a bit of a flag to look at the candidate's resume closely. This is equivalent to choosing to write a Web application in PHP. Sure, you can do it, and it can be fine, but that's definitely not the mainstream solution anymore, and it kind of reeks of choices we all made 15 years ago.
• AOL (e.g., foo@aol.com): This is downright old school. This is kind of like saying you'd still prefer to use .NET 2.0. Well yes, there are still .NET 2.0 applications out there, but that's definitely not current anymore.
• Current employer's work e-mail (e.g., foo@mycurrentcompany.com): This is just plain unprofessional. The candidate is using company resources to conduct a job search. Thanks, but no thanks.

A single e-mail address should never make or break a candidate. It is part of the overall picture that a candidate presents, though, and so it should reflect some awareness of the current environment. Hiring managers expect candidates who keep up with technology changes and trends—even within a language or tool chain—and the e-mail address is a quick, small indicator for seeing how the candidate is keeping up. Make sure your e-mail address says what you want it to say.