<s>
This	O
is	O
a	O
list	B-Application
of	I-Application
mail	I-Application
server	I-Application
software	I-Application
:	O
mail	B-Protocol
transfer	I-Protocol
agents	I-Protocol
,	O
mail	B-Protocol
delivery	I-Protocol
agents	I-Protocol
,	O
and	O
other	O
computer	B-Application
software	I-Application
which	O
provide	O
e-mail	O
.	O
</s>
<s>
All	O
such	O
figures	O
are	O
necessarily	O
estimates	O
because	O
data	O
about	O
mail	B-Protocol
server	I-Protocol
share	O
is	O
difficult	O
to	O
obtain	O
;	O
there	O
are	O
few	O
reliable	O
primary	O
sources	O
—	O
and	O
no	O
agreed	O
methodologies	O
for	O
its	O
collection	O
.	O
</s>
<s>
Surveys	O
probing	O
Internet-exposed	O
systems	O
typically	O
attempt	O
to	O
identify	O
systems	O
via	O
their	O
banner	O
,	O
or	O
other	O
identifying	O
features	O
;	O
and	O
report	O
Postfix	B-Application
and	O
exim	B-Protocol
as	O
overwhelming	O
leaders	O
in	O
March	O
2021	O
,	O
with	O
greater	O
than	O
92%	O
share	O
between	O
them	O
.	O
</s>
