The prior syntax did not seem to permit enough flexibility.
s= Policy Scope (plain-text; OPTIONAL; default is "*"). A colon-
separated list of policy scopes specify which protocols to which
this record applies. Verifiers for a given service type MUST
ignore this record if the appropriate type is not listed.
Currently defined protocol types are as follows:
* matches all unlisted service types
! disavows protocol use
- excludes from policy
This tag is intended to constrain the use of policy for various
transport protocols that may implement, should DKIM be defined by
other protocols in the future. This tag can also disavow use
of specific protocols to repudiate references to this domain.
As example, "s=SMTP:-UUCP:!*" would mean this domain only uses
SMTP and UUCP to exchange messages, but that this policy does not
apply to UUCP.
policy-s-tag = %x73 [FWS] "=" [FWS][exclude|disavow] policy-s-tag-
0*( [FWS] ":" [FWS] policy-s-tag-type )
disavow = "!"
exclude = "-"
policy-s-tag-type = "SMTP" /
x-policy-s-tag-type = hyphenated-word ; for future extension
NOTE WELL: This list operates according to