E-Mail Bouncing Back - Our Comcast IP Address is Blacklisted?

Hello,

Our company has been experiencing a lot of e-mail bounceback issues. At first, we thought it was an issue with our e-mail service provider (1&1 Internet) being blocked, but after working with their techincal support, it seems that our Comcast IP address is being blocked as well. According to MXToolbox, our IP address has been blacklisted by three entities: Spamhaus, SORBS, SEM.

We have completed a thorough check on our systems for any possible malware and are all clean. Here is an example of the error we have received from one of our e-mail bouncebacks:

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A message that you sent could not be delivered to one or more of its recipients. The following addresses failed:

SMTP error from remote server after transfer of mail text:

host

spamsniper2.dynasis.com[216.52.198.242]:

554 Service unavailable; Client host

[mout.perfora.net] blocked by zen.spamhaus.org; http://www.spamhaus.org/query/bl?ip=71.63.170.174

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Our Comcast IP address is 71.63.170.174. Is there any way for us to get delisted, or to get a new IP address from Comcast? This issue is interferring greatly with our e-mail communications.