<s>
Authenticated	B-Protocol
Received	I-Protocol
Chain	I-Protocol
(	O
ARC	O
)	O
is	O
an	O
email	O
authentication	O
system	O
designed	O
to	O
allow	O
an	O
intermediate	O
mail	O
server	O
like	O
a	O
mailing	O
list	O
or	O
forwarding	O
service	O
to	O
sign	O
an	O
email	O
's	O
original	O
authentication	O
results	O
.	O
</s>
<s>
This	O
allows	O
a	O
receiving	O
service	O
to	O
validate	O
an	O
email	O
when	O
the	O
email	O
's	O
SPF	B-Protocol
and	O
DKIM	B-Protocol
records	O
are	O
rendered	O
invalid	O
by	O
an	O
intermediate	O
server	O
's	O
processing	O
.	O
</s>
<s>
DMARC	O
allows	O
a	O
sender	O
's	O
domain	O
to	O
indicate	O
that	O
their	O
emails	O
are	O
protected	O
by	O
SPF	B-Protocol
and/or	O
DKIM	B-Protocol
,	O
and	O
tells	O
a	O
receiving	O
service	O
what	O
to	O
do	O
if	O
neither	O
of	O
those	O
authentication	O
methods	O
passes	O
-	O
such	O
as	O
to	O
reject	O
the	O
message	O
.	O
</s>
<s>
However	O
,	O
a	O
strict	O
DMARC	O
policy	O
may	O
block	O
legitimate	O
emails	O
sent	O
through	O
a	O
mailing	O
list	O
or	O
forwarder	O
,	O
as	O
the	O
DKIM	B-Protocol
signature	O
will	O
be	O
invalidated	O
if	O
the	O
message	O
is	O
modified	O
,	O
such	O
as	O
by	O
adding	O
a	O
subject	O
tag	O
or	O
footer	O
,	O
and	O
the	O
SPF	B-Protocol
check	O
will	O
either	O
fail	O
(	O
if	O
the	O
forwarder	O
did	O
n't	O
change	O
the	O
bounce	O
address	O
)	O
or	O
be	O
aligned	O
with	O
the	O
mailing	O
list	O
domain	O
and	O
not	O
with	O
the	O
message	O
author	O
's	O
domain	O
(	O
unless	O
the	O
mailing	O
list	O
rewrites	O
the	O
From	O
:	O
header	O
field	O
.	O
)	O
</s>
<s>
Even	O
if	O
the	O
SPF	B-Protocol
and	O
DKIM	B-Protocol
validation	O
fail	O
,	O
the	O
receiving	O
service	O
can	O
choose	O
to	O
validate	O
the	O
ARC	O
chain	O
.	O
</s>
<s>
If	O
it	O
indicates	O
that	O
the	O
original	O
message	O
passed	O
the	O
SPF	B-Protocol
and	O
DKIM	B-Protocol
checks	O
,	O
and	O
the	O
only	O
modifications	O
were	O
made	O
by	O
intermediaries	O
trusted	O
by	O
the	O
receiving	O
service	O
,	O
the	O
receiving	O
service	O
may	O
choose	O
to	O
accept	O
the	O
email	O
.	O
</s>
<s>
ARC-Seal	O
(	O
abbreviated	O
AS	O
)	O
-	O
A	O
combination	O
of	O
an	O
instance	O
number	O
(	O
i	O
)	O
,	O
a	O
DKIM-like	O
signature	O
of	O
the	O
previous	O
ARC-Seal	O
headers	O
,	O
and	O
the	O
validity	O
of	O
the	O
prior	O
ARC	O
entries	O
.	O
</s>
