<s>
mail	O
is	O
a	O
command-line	B-Application
email	B-Protocol
client	I-Protocol
for	O
Unix	B-Application
and	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
"	O
Electronic	O
mail	O
was	O
there	O
from	O
the	O
start	O
"	O
,	O
Douglas	O
McIlroy	O
writes	O
in	O
his	O
article	O
"	O
A	O
Research	B-Operating_System
UNIX	I-Operating_System
Reader	O
:	O
Annotated	O
Excerpts	O
from	O
the	O
Programmer	O
’s	O
Manual	O
,	O
1971-1986	O
"	O
,	O
and	O
so	O
a	O
command	B-Application
was	O
included	O
in	O
the	O
first	O
released	O
version	O
of	O
research	B-Operating_System
Unix	I-Operating_System
,	O
First	B-Operating_System
Edition	I-Operating_System
Unix	I-Operating_System
.	O
</s>
<s>
This	O
version	O
of	O
mail	O
was	O
capable	O
to	O
send	O
(	O
append	O
)	O
messages	O
to	O
the	O
mailboxes	O
of	O
other	O
users	O
on	O
the	O
Unix	B-Application
system	I-Application
,	O
and	O
it	O
helped	O
managing	O
(	O
reading	O
)	O
the	O
mailbox	O
of	O
the	O
current	O
user	O
.	O
</s>
<s>
Although	O
initially	O
installed	O
at	O
,	O
(	O
with	O
the	O
earlier	O
Unix	B-Application
mail	O
still	O
available	O
at	O
)	O
,	O
on	O
most	O
modern	O
Unix	B-Application
and	O
Linux	B-Application
systems	O
the	O
commands	O
,	O
and/or	O
all	O
invoke	O
a	O
descendant	O
of	O
this	O
Berkeley	O
Mail	O
,	O
which	O
much	O
later	O
was	O
the	O
base	O
for	O
the	O
standardization	O
of	O
a	O
mail	B-Protocol
program	I-Protocol
by	O
the	O
OpenGroup	B-Application
,	O
the	O
POSIX	O
standardized	O
variant	O
mailx	B-Device
.	O
</s>
