I have been having lots of issues with email sent via Powweb being blacklisted by Spamhaus. Apparently Spamhaus has continuing problems with Spam being generated by Powweb customers and being relayed via Powweb's email servers. Companies that use Spamhaus as part of their spam strategy subscribe to the Spamhaus blacklist and will block email from Powweb servers. ATT and Yahoo are the two Spamhaus users that were blocking my Powweb users email and causing me the most trouble.
I entered a ticket with Powweb and Powweb says that this problem is to be expected "in a shared email hosting environment." Powweb's strategy was to rotate email server IP addresses as soon as SpamHaus blocks one. Unfortunately, this strategy was always one step behind and I found that email was almost never delivered reliably to ATT and Yahoo accounts. I find this email strategy and level of service unacceptable. If reliable email delivery is a requirement for you and your users, you may want to consider alternatives to Powweb for email. There are many alternative providers who seem to have figured out how to deal with SpamHaus and can deliver their users email reliably.