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squirrelmail or other webmail?

I noticed today that squirrelmail is only in Debian oldstable and
oldoldstable.  So,  I'm curious as to its status -- is it now deprecated
(as one would suspect from it not being kept up in more recent Debian
versions)?  Is there some other webmail interface that's being used now?
Note that due to some firewall issues over at the client organization I
can't just use an ldap/smtp server and have users use thunderbird or
something; it has to be a webmail interface.