When agents/publishers ask about previous publishing credits, they don't consider self-publishing a "real" credit, do they? If you have that in your background--and you're sending an electronic query--should you remove the sig line?
You're right; self-publishing isn't considered a "real" credit during the query process.
Here's why: when an agent or publisher asks for publishing credits, what they're really wanting to know is whether you've been through the acceptance and rejection process. Has someone else in the industryevaluated your work and deemed it worthy of publication instead of rejection?
Technically, blogs are "published" online, but you wouldn't include that as a credit, would you? Of course not, because blog posts haven't been vetted through the quality machine.
Similarly, anyone can self-publish a novel. Granted, there are a lot of very good self-published books around (I've read two excellent ones in the last year, and Writer's Digest has their own self-published book awards).
That said, anyone can self-publish, even someone who can't tell a period from a comma. Self-publishing is particularly easy with modern print-on-demand technology.
The upshot is that self-publishing won't tell an editor that you can write.
Now, if your self-published book won a prestigious award (such as the Writer's Digest contest), then it would be worth mentioning.
I don't know whether it would hurt to have a self-published book listed in your e-mail sig line (that might be a question for an agent to answer), but I definitely wouldn't mention it in the body of the query.
Other types of publishing credits are worth mentioning: magazine articles, short stories in anthologies, essays, and so on. Basically, any place where there's an acceptance/rejection system in place. And of course, the bigger the clout and audience of the place that accepted you, the better.