<s>
Smail-3	O
is	O
a	O
mail	B-Protocol
transfer	I-Protocol
agent	I-Protocol
(	O
MTA	O
)	O
used	O
on	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
It	O
is	O
Free	B-Application
Software	I-Application
and	O
is	O
licensed	O
under	O
the	O
GNU	B-License
GPL	I-License
.	O
</s>
<s>
It	O
is	O
still	O
in	O
use	O
on	O
the	O
Internet	O
,	O
but	O
it	O
has	O
been	O
mostly	O
superseded	O
by	O
Exim	B-Protocol
(	O
which	O
was	O
originally	O
based	O
on	O
Smail	B-Protocol
)	O
and	O
other	O
more	O
modern	O
mail	B-Protocol
transfer	I-Protocol
agents	I-Protocol
.	O
</s>
<s>
Smail-3	O
still	O
follows	O
a	O
monolithic	O
design	O
model	O
where	O
a	O
single	B-Application
binary	I-Application
controls	O
the	O
core	O
facilities	O
of	O
the	O
MTA	O
.	O
</s>
<s>
Nevertheless	O
,	O
Smail	B-Protocol
's	O
security	O
record	O
has	O
been	O
fairly	O
clean	O
,	O
and	O
has	O
only	O
been	O
the	O
subject	O
of	O
one	O
BUGTRAQ	O
advisory	O
.	O
</s>
