Having problems with emails

Hello, I work for a company and we do a lot of spam. Lately, we are having a lot of problems with receiving/sending emails. We have a static IP which is not blacklisted, and also i checked the domain names and those are not blacklisted either. when we send emails from the company, we have problems, but when I send the same email from a different internet connection, i have no problems,

any suggestions? Thanks

I forgot to say that when i use my personal gmail account to test it, it doesn't work either; however, when I use my personal gmail from outside, it works. So basically everything in the company doesn't work. I thought it was malware but clean all the computers and still is not working. My second option is to change the IP address...

Yes you need to change your IP address. You IP might be blacklisted as you already mentioned that you do a lot of spam. Are you able to mail while using proxies? Check this thing.

Thanks man! we got a plan with 5 statics IP's.
But I am sure that the problem was AVAST, it was with email filter ON. I am pretty sure that was the problem!

Thanks!!!