I have multiple websites on multiple domains running on the same web server. I have msmtp set up to send email via a special server account on domaina for all sites, and that account has sending permissions for the addresses it needs. However, mail with the from address of domainb gets signed using the key from domaina. Occasionally, the signing will work properly, but that’s the exception. Sending mail directly from a mailbox on domainb results in a properly signed message.
tl;dr: sending mail from email@example.com via the account firstname.lastname@example.org results in the message being signed by domaina.com’s DKIM key, not domainb.com’s DKIM key. Signs with the proper domain only some of the time.