Stop my server from spamming emails

Someone has hacked into my server and caused it to where emails are being sent out from fake email addresses that are using my URL "[login to view URL]" as the domain of the email addresses. They are sending out at a pace of about 30 per minute. This is causing 2 problems -- 1) people are getting spammed from what they think is my website, and 2) my SMTP relays are hitting their daily limit, and it stops me from being able to send out any emails once the daily limit is met. I did find a lot of files that were inserted into my server that I didn't put there, and I have deleted all of the ones I could find, but this issue is still not fixed. I need these emails to stop being sent out immediately. Thanks.