Spam Mail issue (thousands of emails in mail queue)

Yesterday and again today I noticed that there was abnormal amount of emails in the mail queue on our server, there was about 20k emails in there yesterday and again today. I deleted the mail queue clearing them all out but also notice in the catch-all mail account there is thousands of emails in there too, bounce-backs from spam emails.

If someone is sending email from our server I am not sure exactly how they are doing it, I don't see anything obvious at the moment.

What can I check or do to try to figure this problem out? any help would be greatly appreciated.