Building a better list and the quality of your email address are inevitably intertwined.  A recent change at Comcast / Roadrunner may be affecting your deliverability bounce rates. 

Adelphia was a national cable broadband provider that went out of business way back in 2005. Comcast and Time Warner (Roadrunner) bought the assets and split them up between them.

Since then, Comcast and Roadrunner have been slowly transitioning users off of the adelphia.net domain, region by region.

Comcast/Roadrunner are not notifying ISPs or senders when these changes take place. They do notify end recipients whose email addresses are changing with about a month's notice, and encourage them to notify people that their email address is changing.

Comcast is now fully transitioned off of Adelphia, and the Adelphia.net domain will no longer forward mail for Comcast users. That means that you're going to see a higher than average number of bounces at adelphia.net for the few mailings, until these addresses move to undeliverable automatically and scrubbed from future mailings. The ExactTarget system will handle these bounces properly and automatically.

Road Runner, the current owner of the Adelphia.net domain, continues to support email for a large number of users in the Adelphia.net email domain. They have not announced plans to retire the Adelphia.net domain at this time.

Note that for the many email addresses that were just "retired," it's not possible to translate them into comcast.net email addresses, as their username (and therefore the left part of their email address) has likely changed.