<s>
The	O
term	O
Mail	O
Hub	O
is	O
used	O
to	O
denote	O
an	O
MTA	B-Protocol
(	O
message	B-Protocol
transfer	I-Protocol
agent	I-Protocol
)	O
or	O
system	O
of	O
MTAs	O
used	O
to	O
route	O
email	O
but	O
not	O
act	O
as	O
a	O
mail	B-Protocol
server	I-Protocol
(	O
having	O
no	O
end-user	O
email	O
store	O
)	O
since	O
there	O
is	O
no	O
MUA	B-Protocol
(	O
mail	B-Protocol
user	I-Protocol
agent	I-Protocol
)	O
access	O
.	O
</s>
<s>
A	O
first	O
example	O
for	O
a	O
Mail	O
Hub	O
consisting	O
of	O
a	O
network	O
of	O
MTAs	O
would	O
be	O
that	O
of	O
a	O
typical	O
small-to-medium	O
size	O
Internet	O
service	O
provider	O
(	O
ISP	O
)	O
,	O
or	O
for	O
a	O
FOSS	B-License
corporate	O
mail	O
system	O
.	O
</s>
<s>
This	O
is	O
mostly	O
due	O
to	O
not	O
using	O
expensive	O
Network	B-Application
level	I-Application
switches	I-Application
and	O
hardware	O
.	O
</s>
<s>
Simple	O
DNS	B-Protocol
MX	O
record	O
based	O
Mail	O
Hub	O
cluster	B-Architecture
with	O
parallelism	O
and	O
front-end	O
failover	O
and	O
load	O
balancing	O
is	O
illustrated	O
in	O
the	O
following	O
diagram	O
:	O
</s>
<s>
The	O
servers	O
would	O
be	O
all	O
Linux	B-Application
x86	B-Operating_System
servers	O
with	O
low	O
cost	O
SATA	O
or	O
PATA	B-Protocol
hard	O
disk	O
storage	O
.	O
</s>
<s>
The	O
front-end	O
servers	O
would	O
most	O
likely	O
run	O
Postfix	B-Application
with	O
Spamassassin	B-Language
and	O
ClamAV	B-Application
.	O
</s>
<s>
This	O
RAIS	B-Application
server	B-Architecture
Cluster	I-Architecture
would	O
then	O
overcome	O
the	O
problem	O
with	O
Perl	B-Language
based	O
Spamassassin	B-Language
being	O
too	O
CPU	O
and	O
memory	O
hungry	O
for	O
low	O
cost	O
servers	O
.	O
</s>
<s>
The	O
solution	O
presented	O
here	O
is	O
based	O
on	O
all	O
GPL	B-License
FOSS	B-License
free	O
software	O
,	O
but	O
of	O
course	O
there	O
are	O
alternative	O
configurations	O
using	O
other	O
free	O
or	O
non-free	O
software	O
.	O
</s>
