Sending email to all your contacts about your new book?
Including all the agents you queried before you were published?
I don't know how other agents deal with this, but when you send me an email like that, I add your email address to my spam filter so I never have to hear from you again.
This might not be what you want to accomplish.
At some point in the future you might want to get in touch with me. You might want one of my clients to blurb your book, or invite me to your conference, or even ask a question for Friday Night at the Question Emporium. If your email is listed as spam, it doesn't go to my spam folder. It gets discarded. I'll never see it.
It's a Very Good Idea to use a mail service to send your promotional emails. At the bottom of those emails are instructions on how to
unsubscribe. If you send me an announcement with an unsubscribe
button, I'll click that rather than listing your email as spam.