Absolutely agree with GuyverII--I find people who sign their e-mails with

XXXX, Ph.D.

must have some inferiority complex. If you are at a respectable academic institution, everyone knows that you have a Ph.D. (or in a handful of cases such as Sandy Jencks at Harvard, you've outpublished almost everyone even without a Ph.D.).

But I don't use Dr. with my colleagues--we all go by first names. I have been tempted (and may have yielded to it, but I can't remember) to use Dr. when an MD calls me by my first name as in, "Ric, this is Dr. Jones."