It doesn't look like att.blackberry.net is configured to receive mail yet.. who knows when:
$ host -t mx att.blackberry.net
Host att.blackberry.net not found: 3(NXDOMAIN)
As compared to:
$ host -t mx mycingular.blackberry.net
mycingular.blackberry.net mail is handled by 10 mx02.bis.na.blackberry.com.
mycingular.blackberry.net mail is handled by 10 mx03.bis.na.blackberry.com.
mycingular.blackberry.net mail is handled by 10 mx04.bis.na.blackberry.com.
mycingular.blackberry.net mail is handled by 10 mx01.bis.na.blackberry.com.