Best Email Security Tool

Use cloud mail security. It will work significantly better at protecting your environment than anything on-prem or installed on the server itself. There are several companies that provide these services. For SMB, Barracuda is usually good from a security vs usability standpoint (it's easy to config and work with, but is not as customizable as others). Exchange Online Protection is also a good solution. Mimecast, Proofpoint, Fortinet, and several others are also great solutions.

Receiving a spam to your email users and be a blocked by security testing services is a 2 different cases.
You've bee blocked because your users are sending spam or your server is not secured by any available encryption keys like DMARK or  DKIM and passwords for your users are week and someone is using your server to send spam out !!

1. First, Implement DKIM or DMARK on your server and edit your public DNS and upgrade it with this KEY to be sure that only your server can send email

2. Force your users to change passwords using more complecity
3. Enable spam filtering on your Exchange

4. If you want you can pay some company to filter spam for you. I can give you access to my good friend who providing that kind of service and is not expensive. I'm using it for a years and my spam my users are getting was cat off about 95%

You can try Anti-spam and anti-malware protection by Microsoft.
https://docs.microsoft.com/en-us/exchange/anti-spam-and-anti-malware-protection-exchange-2013-help 

or you can use another cloud base email security gateway solution like Barracuda, Symentec...

1) receiving lot of spam emails

Has to do with incoming mail.

Fairly easy to fix this by using a service like https://SpamHero.com or running https://rspamd.com connected to your MTA.

2) and our mail server IP started blacklisted in some places.

This has to do with outgoing mail.

If you're sending Spam, you will be blacklisted.

Fix: Stop sending Spam.

Now you might say... "Whoa! I never send Spam!"

What you believe + how Mailbox Providers (Google/Yahoo/Microsoft/etc...) classify your mail are 2x different views.

Tip: If you're getting blacklisted, then you're sending Spam... from the Mailbox Provider perspective...

The way you "Stop sending Spam" is to review all your outgoing MTA logs + resolve any SMTP responses that aren't "250 Success".

Any non-250 response, you'll take action about + fix.

3) You can also tie in rspamd + many common content scanning services to block any outgoing email which might be flagged as Spam.

This might be helpful also.

Note: This can take a very long time too.

My company is using Sonicwall ES 7000

Hi Ram,

I may be a little late to the table on this one but I represent www.spambrella.com a Proofpoint service. If it helps I would be happy to discuss your spam pain points and business requirements to align the service most suitable for your needs. We have pre-packaged services for any sized organization so I am sure we will be a good fit for you in any capacity.

Let me know if I can help?
Thanks!

Dears,

anyone using Barracuda Email Security - can I know the feedback and their reputation

Thanks

-->anyone using Barracuda Email Security - can I know the feedback and their reputation
One of my customer is using without issue. He activated sandboxing as well.

Hi Ram,
I have used it 2 years ago in my previous job without any issue.
You can see reviews in below link,
https://www.gartner.com/reviews/market/email-security/vendor/barracuda/product/barracuda-email-security-gateway 

I recommend to assign points to all experts equally, as lot of options are provides by us(Expert) to the user here.