Hi Vivek, 

I'm sorry we didn't see this earlier. Our Knowledge Base contains an article that will help in these scenarios. 

Please let me know if this helps! 

Vivek, in addition to what Andrea provided, I recommend looking for compromised accounts and resolving those as well:

https://portal.smartertools.com/kb/a2873/find-a-compromised-account.aspx

Additionally, please make sure your domains and Settings>Protocols>SMTP In has been configured to require SMTP Authentication as your server may be acting as an open relay as well. Hope that helps too!

Hi Vivek. If you would like to protect your server in the event of a compromised account, SM's throttling and internal spammer notification features work well for that along with the free program that my company, Mail's Best Friend, offers FREE of charge. The program is called Declude and the feature within it that you would use is Declude Hijack. If you would like to learn more about how it works, Please check out the following KB articles and user's manual...

Also, if you would like to download and try Declude, you can do so from the following link: http://mailsbestfriend.com/downloads/

I hope this helps. Thanks.