Any service that uses OpenSSL for its TLS implementation is potentially vulnerable; this is a weakness in the underlying cyrptography library, not in how it's presented via a web server or email server package. You should consider all linked services vulnerable to data leakage at least.
As I'm sure you're aware, it's quite possible to chain attacks together. Even in the simplest attacks it's perfectly possible to, for example, use Heartbleed to compromise SSL, read webmail credentials, use webmail credentials to gain access to other systems with a quick "Dear helpdesk, can you give me a new password for $foo, love CEO".
There's more information and links in The Heartbleed Bug, and in another question maintained by a Server Fault regular, Heartbleed: What is it and what are options to mitigate it?.
apt-get updateshould be enough for Ubuntu now without downgrading, the patch appeared in the main repository last night.