mcabber
mcabber is a console based Jabber/XMPP client. It supports GPG and OTR end to end encryption. It also supports XEP-0280 - Message Carbons, so it will receive all messages if you use multiple clients (which must all support XEP-0280).
A default configuration file is included with mcabber, and you can also check out the latest defaults on github. Here are the settings you should use for jabber.at:
# Your full JID
set jid = user@jabber.at
# Set the password if you wish, otherwise mcabber will prompt you
#set password = ...
# General security settings (These are the default anyway)
set tls = 1
set ssl_ignore_checks = 0
# We don't recommend you set the Fingerprint: Since we use Let's Encrypt,
# certificates are renewed every two to three months.
#set ssl_fingerprint = ...
# DO NOT SET THIS: mcabber is able to figure out server and port by itself.
#set server = ...
#set port = ...
mcabber APT repository
If you use Debian/Ubuntu, we maintain an APT repository with the newest version of mcabber.