You can disable this check, but unfortunately I do not know of a way to simply whitelist a domain.
Here is an old KB article written for the 6.x version that still applies to newer versions (I am sorry about the formatting):
Sidewinder G2: ERROR: Message-Id header line format error (when email is rejected)
The Sidewinder is rejecting the email because it is not compliant with the applicable Internet Engineer Task Force's Request For Comments, specifically RFC2821 and RFC2822. From RFC2821:
The message identifier (msg-id) itself MUST be a globally unique
identifier for a message. The generator of the message identifier
MUST guarantee that the msg-id is unique. There are several
algorithms that can be used to accomplish this. Since the msg-id has
a similar syntax to angle-addr (identical except that comments and
folding white space are not allowed), a good method is to put the
domain name (or a domain literal IP address) of the host on which the
message identifier was created on the right hand side of the "@", and
put a combination of the current absolute date and time along with
some other currently unique (perhaps sequential) identifier available
on the system (for example, a process id number) on the left hand
side. Using a date on the left hand side and a domain name or domain
literal on the right hand side makes it possible to guarantee
uniqueness since no two hosts use the same domain name or IP address
at the same time. Though other algorithms will work, it is
RECOMMENDED that the right hand side contain some domain identifier
(either of the host itself or otherwise) such that the generator of
the message identifier can guarantee the uniqueness of the left hand
side within the scope of that domain.
Semantically, the angle bracket characters are not part of the
msg-id; the msg-id is what is contained between the two angle bracket
The other "gateway" (i.e. the one that sent the email) is violating the message identifier requirements of this section of RFC2821.
A compliant message identifier should look similar to this:
Sidewinder's Sendmail is operating in an RFC compliant manner. If possible, you should contact the sending gateway's administrator to correct the formatting of the message identifier. Should you desire to intentionally violate the RFC, you can comment out the rule "R$* $#error $: 553 Message-Id header line format error" which outputs the error message. This rule can be found in the SlocalCheckMessageId ruleset in the external M4 Config File (Services Configuration, Servers, Sendmail, Configuration tab). Such a modification is unsupported; performing and maintaining such a modification would be your responsibility.
Thanks for your help.
I have comment out the rule "R$*.