<s>
Citadel/UX	B-Protocol
(	O
typically	O
referred	O
to	O
simply	O
as	O
"	O
Citadel	B-Protocol
"	O
)	O
is	O
a	O
collaboration	O
suite	O
(	O
messaging	O
and	O
groupware	B-Application
)	O
that	O
is	O
descended	O
from	O
the	O
Citadel	B-Protocol
family	O
of	O
programs	O
which	O
became	O
popular	O
in	O
the	O
1980s	O
and	O
1990s	O
as	O
a	O
bulletin	B-Application
board	I-Application
system	I-Application
platform	O
.	O
</s>
<s>
It	O
is	O
designed	O
to	O
run	O
on	O
open	O
source	O
operating	O
systems	O
such	O
as	O
Linux	B-Application
or	O
BSD	B-Operating_System
.	O
</s>
<s>
Although	O
it	O
is	O
being	O
used	O
for	O
many	O
bulletin	B-Application
board	I-Application
systems	I-Application
,	O
in	O
1998	O
the	O
developers	O
began	O
to	O
expand	O
its	O
functionality	O
to	O
a	O
general	O
purpose	O
groupware	B-Application
platform	O
.	O
</s>
<s>
In	O
order	O
to	O
modernize	O
the	O
Citadel	B-Protocol
platform	O
for	O
the	O
Internet	O
,	O
the	O
Citadel/UX	B-Protocol
developers	O
added	O
functionality	O
such	O
as	O
shared	O
calendars	O
,	O
instant	O
messaging	O
,	O
and	O
built-in	O
implementations	O
of	O
Internet	O
protocols	O
such	O
as	O
SMTP	B-Protocol
,	O
IMAP	B-Protocol
,	O
Sieve	O
,	O
POP3	B-Protocol
,	O
GroupDAV	O
and	O
XMPP	B-Protocol
.	O
</s>
<s>
All	O
protocols	O
offer	O
OpenSSL	B-Language
encryption	O
for	O
additional	O
security	O
.	O
</s>
<s>
Users	O
of	O
Citadel/UX	B-Protocol
systems	O
also	O
have	O
available	O
to	O
them	O
a	O
web-based	O
user	O
interface	O
which	O
employs	O
Ajax	B-General_Concept
style	O
functionality	O
to	O
allow	O
application-like	O
interaction	O
with	O
the	O
system	O
.	O
</s>
<s>
Citadel	B-Protocol
uses	O
the	O
Berkeley	B-Language
DB	I-Language
database	O
for	O
all	O
of	O
its	O
data	O
stores	O
,	O
including	O
the	O
message	O
base	O
.	O
</s>
<s>
Citadel/UX	B-Protocol
became	O
free	B-License
and	I-License
open-source	I-License
software	I-License
subject	O
to	O
the	O
terms	O
of	O
the	O
GPL-2.0-or-later	B-License
license	O
in	O
1998	O
.	O
</s>
<s>
In	O
2006	O
Citadel	B-Protocol
was	O
relicensed	O
to	O
the	O
GPL-2.0-only	B-License
license	O
.	O
</s>
<s>
In	O
2007	O
Citadel	B-Protocol
was	O
relicensed	O
again	O
to	O
the	O
GPL-3.0-only	B-License
license	O
.	O
</s>
