We have seen a dramatic increase in companies being Blacklisted recently. This means that your email may be shut off at the ISP level. Incoming mail may still work, but anything outgoing may stop. You could also be shut off for ALL internet connectivity. 
The cause of this is a large amount of data coming OUT of your companies internet address, or external IP address. The 2 main reasons for this are:
 1. An internal pc or computer that has a virus that is sending a large anount of data to the internet. It could also be contacting a number of malware sites, or sites that have known issues.
 2. A large amount of mail, (500 +), going out to the internet from the company. If you use mass email marketing tools, this could be the cause. A large amount of data sent in emails could also set off the the blacklist status.
 There are a handful of sites that manage these lists, and some of them can be difficult to track down. In any case, you must submit a ticket for them to UN-list you, and then wait from 12 hours to 3 days before they restore service. If you get blacklisted again, they may shut down everything. It will be more difficult to restore service at this point.
 If you have this issue, or see it in the future, please call us if you would like assistance. We can assist with the process, or provide tools that monitor for this.
 Thanks ,
 Holland Computer Support Team.