apt: how to search using package tags?

Some (most?) packages in Debian, have tags. For example:

=# apt-cache show squirrelmail | perl -lne 'print if /^Tag:/.../^\S/'
Tag: implemented-in::php, interface::web, mail::imap, mail::smtp,
 mail::user-agent, network::server, protocol::http, protocol::imap,
 protocol::smtp, role::program, scope::application, use::browsing,
 use::editing, use::searching, use::transmission, use::viewing,
 web::TODO, web::application, works-with::mail
Section: web

But, I can't find a way, to list all packages with given tag. Is it possible?

You probably want to take a look at all the documentation surrounding Debtags. It's... not as simple as you might think. No, I don't know why, either.

I wonder why people haven't answered:

aptitude search '?tag(protocol::dns)'

aptitude search allows complex search queries with probes on different package description fields and logical operators:

aptitude search '?tag(game::strategy) ?installed'
aptitude search '?tag(protocol::dns) !?installed'

Refer to Search term reference in Aptitude User's Manual from aptitude-doc-en package (/usr/share/doc/aptitude/html/en/index.html).