If DKIM signing is enabled (DKIM_SIGN_ALL=1), a message that already has a DKIM signature when it arrives at LISTSERV will have that signature replaced by one generated by LISTSERV.

LISTSERV can be configured to retain the old signature of such messages via the list -level keyword setting “Misc-Options= KEEP_DKIM_SIGNATURE”, though (as noted above) this is rarely, if ever, recommended. With this setting, only the original signature will be included in the distributed message. LISTSERV won’t add its own signature in this case, as double DKIM signatures are disallowed in most cases and, even when allowed, may not be handled correctly by all implementations.

DKIM can be used to sign mail-merge messages, but in that case LISTSERV’s Embedded Mail Merge (EMM) feature MUST be enabled. Using EMM is the only way to guarantee that the signing engine will see the exact text being sent to the recipient, and that the signature will match.  EMM is normally enabled by default, but can be disabled at the server level.  Check the setting of EMBEDDED_MAIL_MERGE= in the site configuration if you believe this may be an issue.